We granted en banc review in this appeal to determine whether our prior decision in United States v. Dahms, 938 F.2d 131 (9th Cir.1991), should be overturned.1 Dahms looks to state law to determine the scope of restrictions on firearms possession under federal statutes governing the possession of firearms by previously convicted felons. The question typically arises when a previously convicted felon is granted a restoration of civil rights by state authorities including a partial restoration of the right to possess firearms.
Other circuits have adopted an “all or nothing” approach, holding that unless a state grants a complete restoration of the right to possess all firearms, the individual remains subject to federal prosecution as a felon in possession—even for a firearm that the former felon has an express right to possess under state law. See United States v. Estrella, 104 F.3d 3, 7-9 (1st Cir.), cert. denied, — U.S. -, 117 S.Ct. 2494, 138 L.Ed.2d 1001 (1997);2 United States v. Burns, 934 F.2d 1157, 1160 (10th Cir.1991); United States v. Driscoll, 970 F.2d 1472, 1480 (6th Cir.1992); accord, United States v. Lee, 72 F.3d 55, 57-58 (7th Cir.1995) (absent full restoration of firearms rights, civil rights not restored at all); United States v. Ellis, 949 F.2d 952, 955 (8th Cir.1991) (same).
We respectfully disagree with our sister circuits. We believe the language of the statute evidences at least an ambiguity which must be resolved in Qualls’ favor and more likely an intent to defer to state law concerning the nature of the restoration of a former felon’s civil rights (including the right to possess firearms).
I.
Congress has made it a federal crime for a previously convicted felon to possess a firearm. Under what has become known as the felon-in-possession statute:
*826It shall be unlawful for any person ... who has been convicted in any court of, a crime punishable by imprisonment for a term exceeding one year ... to ... possess in or affecting commerce, any firearm____
18 U.S.C. § 922(g)(1).
Congress has provided an exception in section 921(a)(20) to the prohibition which is at the heart of the issue before us:
What constitutes a conviction [of a felony] shall be determined in accordance with the law of the jurisdiction in which the proceedings were held. Any conviction ... for which a person ... has had civil rights restored shall not be considered a conviction for purposes of this chapter, unless such ... restoration of civil rights expressly provides that the person may not ship, transport, possess, or receive firearms.
18 U.S.C. § 921(a)(20) (emphasis added).
It is not uncommon, particularly in this region, for a state to restore some civil rights—such as the right to vote—to a person with a prior felony conviction while, at the same time, partially restoring the right to possess firearms (for example allowing hunting rifles but not pistols or other concealable firearms). Restoration normally occurs upon a successful petition or application of the felon, but it also may occur, as it did here, by operation of law.3
In Dahms, we determined that a state’s classification of a crime, as well as its determination of the scope of the restoration of a prior felon’s civil rights, must be accorded deference. See Dahms, 938 F.2d at 133; see also 18 U.S.C. § 921(a)(20). We therefore held that the prohibition of section 922(g)(1) does not apply to a former felon who possesses a firearm which is allowed to him by state law, even though the state restricted his possession of another type of firearm. See Dahms, 938 F.2d at 134. In so deciding, this court deferred to state law’s determination of the scope of the restoration of a prior felon’s civil rights.4
After careful examination of our prior case law and the rationale of the decisions of our sister circuits, we reiterate our conclusion in Dahms: “To apply § 922(g)(1) and conclude that [a prior felon’s] right to possess any firearm [is] restricted under federal law because the state restricted his possession of [one type of firearm] would undermine the explicit deference to state law in § 921(a)(20).” Id. at 135.
Our reading of the statutes at issue suggests that Congress did not intend this deference to state law to be either incomplete or selective. Contrary to the reasoning of those courts that have adopted an all-or-nothing approach, the exception does not provide for a resort to federal law if state law in any way limits the prior felon’s possession of firearms. While at least one court has justified this as a response to the felon’s failure to overcome the presumption of a federal ban on possession of firearms,5 we view it as running contrary to a rather clear Congressional intent to defer to state law.
We reach our conclusion by examining “the language of the governing statute, guided not by ‘a single sentence or member of a sentence, but looking] to the provisions of the whole law, and to its object and policy.’” John Hancock Mut. Life Ins. Co. v. Harris Trust & Sav. Bank, 510 U.S. 86, 94-95, 114 S.Ct. 517, 523, 126 L.Ed.2d 524 (1993) (quoting Pilot Life Ins. Co. v. Dedeaux, 481 U.S. 41, 51, 107 S.Ct. 1549, 1555, 95 L.Ed.2d 39 (1987) (listing citations)).
Congress’s intent to rely on state law is apparent from the exception in section *827921(a)(20). In crafting the exception, it is clear that Congress understood that reliance on state law could produce different outcomes in different states and was aware of the divergent results that could result from application of the exception in determining what constitutes a conviction.6 Congress nevertheless determined that deferral to state law was appropriate. There is no suggestion that Congress intended that the deference to state law be selective, allowing varying state interpretations as to what constitutes a felony conviction, while ignoring individual state determinations concerning the restoration of a former felon’s civil rights.
The statute expressly states that “the law of the jurisdiction in which the proceedings were held,” California state law in this case, determines what constitutes a conviction for purposes of the federal prohibition. 18 U.S.C. § 921(a)(20). The statute goes on to say that a past conviction must not be considered if the person’s civil rights were restored, unless “such” restoration “expressly provides” for certain restrictions on firearms. The word “such” tells us “what to read to look for qualifications on a felon’s restoration of civil rights.” United States v. Herron, 45 F.3d 340, 342 (9th Cir.1995). We are to look to the state certificate if there is one, and to state law if the restoration is by operation of law. See id. The reason we look to state law is that Congress, by using the word “such,” referred the courts to the restoration of civil rights, and that restoration is accomplished by the states, not the federal government.
We believe Senator Hatch expressed Congressional sentiment in his statement:
[The exception in section 921(a)(20) ] grants authority to the ... (State) which prosecuted the individual to determine eligibility for firearm possession after a felony conviction____ Since the Federal prohibition is triggered by the States’ conviction, the States’ law as to what disqualifies an individual from firearms use should govern.
131 Cong. Rec. S8686-01 (daily ed. June 24, 1985) (statement of Sen. Hatch) (emphasis added).7 To say that this statement demonstrates merely an intent to defer to state law only if it defines the lawful possession of firearms in a certain way slices the issue far too thin. Such an argument, if accepted, would substitute a federal presumption for the states’ own determinations of firearm eligibility when the statute says just the opposite.
We believe Congress meant what it said with the statement that “[sjtates’ law as to what disqualifies an individual from firearms use should govern.” Congress easily could have mandated resort to the federal ban on possession of firearms if state law in any way limited former felons’ possession. We do not take lightly its policy determination, particularly one made in the face of arguments by federal agencies that this would result in the Balkanization of felon-in-possession enforcement. By striking the balance it did, Congress implicitly recognized that what may make sense in the wilds of Alaska might not on the streets of New York. In any event, we decline to write something into the statute which Congress has made a deliberate policy decision to leave out. We are statute eonstruers, not statute writers.
Finally, we are not free to read into the statute an intent to displace state law whenever a state allows a former felon to possess one type of firearm but not another. To do so would squarely contradict the rule that Congress must be “reasonably explicit” in its intent to impinge upon important state interests. BFP v. Resolution Trust Corp., 511 U.S. 531, 544, 114 S.Ct. 1757, 1764-65, 128 L.Ed.2d 556 (1994) (quoting F. Frankfurter, Some Reflections on the Reading of Statutes, 47 Colum. L.Rev. 527, 539-40 (1947), and citing Kelly v. Robinson, 479 U.S. *82836, 49-50 n. 11, 107 S.Ct. 353, 360-61 n. 11, 93 L.Ed.2d 216 (1986)).8
Without doubt, a state’s interest in determining the scope of permissible firearm possession by persons convicted in its courts is an important one. Application of an all-or-nothing approach not only intrudes upon that interest, but also acts to substitute a federal definition of lawful firearm possession for that of the state. This is precisely the uniform prosecutive standard that federal agencies sought when this statute was enacted and exactly what Congress elected not to provide.
II.
Even if we were to read the language and policy of the exception in section 921(a)(20) as ambiguous, the Rule of Lenity9 would mandate that we construe the exception as requiring that we defer to state law’s determination of the scope of the restoration of a prior felon’s civil rights. The Rule instructs that when construing an ambiguous criminal statute, courts must “infer the rationale most favorable to the [defendants].... ” United States v. Martinez, 946 F.2d 100, 102 (9th Cir.1991); accord Liparota v. United States, 471 U.S. 419, 427, 105 S.Ct. 2084, 2089, 85 L.Ed.2d 434 (1985).
Applying the Rule to section 921(a)(20) compels the conclusion that the exception’s deference to state law is complete and that state law controls the scope of restoration of civil rights, including which firearms a former felon may possess. To interpret the statute in any other manner would subject persons to the federal ban against possession of firearms in a state which has authorized the possession of the very type of firearm for which prosecution is sought. This Rule operates with particular force here where Qualls could not only reasonably rely on state law as to which firearms he might lawfully possess, but also the interpretative law of the federal circuit {Dahms) where the firearm was possessed.
III.
Because the district court denied Qualls’ request for a jury instruction limiting the firearms to be considered to those which California law prohibited him from possessing,10 our review is de novo.11 See United *829States v. Eshkol, 108 F.3d 1025, 1028 (9th Cir.), cert. denied, —U.S.-, 118 S.Ct. 120, 139 L.Ed.2d 71 (1997) (“We review de novo a denial of a defendant’s jury instruction based on a question of law.”).
We examine the scope of a former felon’s right to possess firearms by reference to the state law at the time his civil rights were restored, without regard to a later restriction or expansion of the defendant’s civil rights. See United States v. Collins, 61 F.3d 1379, 1382 (9th Cir.1995). We agree with the panel’s determination, see United States v. Qualls, 108 F.3d 1019, 1023 (9th Cir.1997), that because the scope of Qualls’ civil rights was determined by the statutes under which he was convicted and those rights were never impaired, we look to the state law at the time of his conviction because it is analogous to the time of restoration. At the time of his conviction in 1975, California law prohibited Qualls from possessing pistols, revolvers, and other concealable weapons, but allowed the possession of long guns. See Cal.Penal Code § 12021(a). Qualls’ 1994 indictment charged him with one count of being a felon in possession of the following seven firearms: two revolvers, one pistol and four rifles. The jury was required to find that Qualls possessed at least one of these firearms, and jury unanimity was required as to which firearm(s) Qualls possessed; however, the jury was not required to specify which firearm(s) it found Qualls possessed. The jury ultimately found Qualls guilty of being a former felon in possession of a firearm in violation of 18 U.S.C. § 922(g)(1), and he was sentenced to forty-one months imprisonment.
As did the panel, we hold that the district court erred in failing to instruct the jury that Qualls could only be convicted for possessing concealable weapons as prohibited in Cal.Penal Code § 12001(a). Because the jury was not given the opportunity to identify which weapon(s) it found Qualls possessed, we cannot determine whether the jury found Qualls guilty of possessing a concealable weapon or some other weapon. If the jury found that Qualls possessed a non-concealable weapon and convicted him on that ground, his conviction was improper because, as a matter of law, the jury was not permitted to convict him on that basis.12 Cf. United States v. Barona, 56 F.3d 1087, 1097 (9th Cir.1995), cert. denied, 516 U.S. 1092, 116 S.Ct. 814, 133 L.Ed.2d 759 (1996) (“The problem in this case ... is that, among the list of people who the jury was told that it could choose, there existed individuals that the jury was not allowed to choose as a matter of law.”).
As we explained in Barona, the Supreme Court has determined that a verdict must be set aside in cases such as this where the verdict is legally insupportable on one ground, yet supportable on another, and it is impossible to tell on which ground the jury relied. See id. at 1097-98 (relying on Yates v. United States, 354 U.S. 298, 77 S.Ct. 1064, 1 L.Ed.2d 1356 (1957), and the Supreme Court’s interpretation of Yates in Griffin v. United States, 502 U.S. 46, 112 S.Ct. 466, 116 L.Ed.2d 371 (1991)). It is impossible to determine whether the jury convicted Qualls of the possession of a weapon he had a perfect right to possess in California.
*830We are unable to engage in a harmless error analysis in this case because “it would be virtually impossible to determine whether the [trial court error] was harmless enough to warrant affirming the conviction.” United States v. Annigoni, 96 F.3d 1132, 1144 (9th Cir.1996) (en banc). See generally United States v. Bauer, 132 F.3d 504, 510 (9th Cir.1997) (holding that where error is nonconstitutional, the Government must show that the resulting prejudice “was more probably than not harmless,” i.e., the Government must show “a ‘fair assurance’ that the verdict was not substantially swayed by the error”). Because we can only speculate whether the improper denial of the defendant’s proposed jury instructions caused a prejudicial error or was merely harmless, harmless error review is inapplicable. See Annigoni, 96 F.3d at 1144-45.
The verdict and resulting judgment of conviction must be set aside.
REVERSED.
. To the extent not inconsistent with this opinion, we adopt the facts as set forth in the panel opinion. See United States v. Qualls, 108 F.3d 1019 (9th Cir.1997).
. The Supreme Court recently granted certiorari in United States v. Caron, Nos. 96-2338 and 96-2339, unpubl. order (1st Cir. May 9, 1997), cert. granted, Caron v. United States,-U.S.-, 118 S.Ct. 680, 139 L.Ed.2d 628 (1998). In Caron, the First Circuit relied on the "all or nothing” rule of Estrella, which holds that a conviction counts for purposes of § 921(a)(20) if the state continues to restrict significantly a prior felon’s firearm use, to reverse the district court's determination in United States v. Caron, 941 F.Supp. 238 (D.Mass.1996). The district court held that a prior conviction did not count as a predicate crime under 18 U.S.C. § 921(a)(20) for a previously convicted felon who was found in possession of rifles and shotguns. Massachusetts law gave him the right to possess, transport and ship rifles and shotguns, but not to cariy handguns outside his home or business. See id. at 246-56.
.Defendant Danny Qualls ("Qualls”) was convicted in 1975 for assault with a deadly weapon and sentenced to a period of probation which was suspended and later abated upon payment of probation costs. See Qualls, 108 F.3d at 1021. Both sides agree, for the purposes of this appeal, that this is the functional equivalent of Qualls having applied for and received a restoration of his civil rights, including the right to possess firearms not capable of being concealed on his person.
. Qualls was found in possession of several firearms, including those permitted by California law and those not so permitted. In the proceedings below, the issue was submitted to the trial jury in a manner that did not require jurors to identify which firearms Qualls possessed. See Qualls, 108 F.3d at 1024.
. See Driscoll, 970 F.2d at 1480.
. See, e.g„ H.R.Rep. No. 99-495, at 1342 (1986), Letter from Director of Treasury, Bureau of Alcohol, Tobacco and Firearms (warning Congress that "The bill provides that what constitutes a felony conviction would be determined by the law of the jurisdiction where the conviction occurred. This would require the Bureau to examine the peculiar laws of each State to determine whether a person is convicted for Federal purposes.").
. Senator Hatch introduced the bill, along with Senators McClure, Symms, Denton and Thurmond.
. See also City of Burbank v. Lockheed Air Terminal, Inc., 411 U.S. 624, 633, 93 S.Ct. 1854, 1859, 36 L.Ed.2d 547 (1973) ("[W]e start with the assumption that the historic police powers of the States [a]re not to be superseded by [a federal statute] unless that was the clear and manifest purpose of Congress.”) (quoting Rice v. Santa Fe Elevator Corp., 331 U.S. 218, 230, 67 S.Ct. 1146, 1152, 91 L.Ed. 1447(1947)).
. The rule is to be resorted to only when “there is a grievous ambiguity or uncertainty in the language and structure of [a statute], such that even after a court has seized everything from which aid can be derived, it is still left with an ambiguous statute.” Chapman v. United States, 500 U.S. 453, 463, 111 S.Ct. 1919, 1926, 114 L.Ed.2d 524 (1991) (internal quotations, citations and alterations omitted).
. Although our prior opinion in Qualls stated that plain error review was appropriate because Qualls had not objected to the proposed jury instructions, see Qualls, 108 F.3d at 1024, careful review of the record reveals that he did, in fact, object:
Court: Let me start with the jury instruction first and find out from defendant if there was any objection to the government's proposed instructions....
Defendant’s Attorney: The Instruction ... doesn’t ... accurately reflect the law.
U.S. v. Dahm [sic] ... would cause California’s law to become controlling in this federal trial. And California’s law ... is that a person can ... earn himself a misdemeanor after satisfactory probation; judgment is withheld for that purpose. And if for some reason the misdemeanor isn't earned and that’s not granted, all civil rights are returned in California at the termination of probation except the right to own or possess a concealable weapon. Long guns can be possessed in California and would supersede the government code.
That is not reflected in the instruction and is contrary to that. It only reflects the federal rule, which is outdated in California due to the Dahms case.
Government: [Tjhe defense counsel gave us a copy of his proposed instruction[ ] ... [which] I believe ... is a concealable weapon instruction on his part. And if that’s what it is, I object to that.
Defendant's Attorney: [A]ll of my client’s rights were restored, including the right to possess guns but not concealable weapons.
That’s why, in the alternative, I have placed ... those two instructions before the Court. *829One defining what a concealable firearm is, and the other one modifying the ... proposed Instruction 16, so that it would fit that definition.
In an Order dated January 18, 1995, the district court denied defendant’s proposed version of the government’s proposed instruction 16 regarding the essential elements of the offense charged and denied defendant’s proposed supplementary jury instruction.
. The Supreme Court’s recent decision in Johnson v. United States, - U.S. -, -, 117 S.Ct. 1544, 1549, 137 L.Ed.2d 718 (1997), is inapplicable. In Johnson, the Court explained that a "plain error” affecting substantial rights can be corrected by an appellate court when there was no objection to the error at trial. Because Qualls objected to the trial court’s jury instruction error, plain error analysis does not apply.
. Qualls’ civil rights were restored by operation of law, and the law provided that he could possess long guns. For him to be convicted of feloniously possessing guns under federal law after his restoration, the state would have had to tell him "point blank that weapons are not kosher.” United States v. Erwin, 902 F.2d 510, 512-13 (7th Cir.1990). Qualls was told by the state law that concealable guns were not kosher but long guns were, so he was entitled to a jury instruction that would allow the jury to convict him only if it concluded that he possessed the handguns charged.