Defendant appeals a judgment of conviction for possession of a weapon by an inmate. ORS 166.275. He challenges the validity of the conviction on the ground that the trial court accepted his no contest plea and sentenced him without requiring the court-appointed Spanish interpreter to place her credentials on the record. He also challenges the validity of the sentence, which imposes an obligation to pay a unitary assessment and attorney fees, on the ground that the trial court failed to make a finding as to defendant’s ability to pay. We affirm.
Defendant requested a Spanish interpreter. The trial court appointed an interpreter and took her oath to provide a true and impartial translation of the proceedings. The court did not require the interpreter’s credentials to be entered into the record. Through the interpreter, defendant entered a plea of no contest to the charge that, on or about August 22, 1996, while committed to a state penal institution, defendant possessed a weapon.
At the sentencing hearing, defendant again requested an interpreter, and the trial court again appointed one. At the sentencing hearing, however, the interpreter took no oath and again did not place her credentials on the record. The trial court imposed a unitary assessment of $94, plus $300 in attorney fees. Defendant entered no objection to the award of attorney fees.
On appeal, defendant contends that his conviction must be set aside because the trial court failed to comply with the requirements of ORS 45.275(7) in accepting his plea and in sentencing him. That statute provides:
“Any person serving as an interpreter for the court in a civil or criminal proceeding shall state or submit the person’s qualifications on the record unless waived or otherwise stipulated to by the parties or counsel for the parties. An interpreter for the court shall swear or affirm under oath to make a true and impartial translation of the proceedings in an understandable manner using the interpreter’s best skills and judgment in accordance with the standards and ethics of the interpreter profession.”
*185According to defendant, at the plea hearing, the trial court erred in failing to place the interpreter’s credentials on the record. He also contends that, at the sentencing hearing, the trial court erred in failing either to swear in the interpreter or to place her credentials on the record. The state contends that defendant waived both requirements.
We need not address either of the parties’ contentions as to the trial court’s failure to comply with the requirements of the statute, because the assignment is not reviewable. ORS 138.050(1) provides that a defendant who has pleaded guilty or no contest may take an appeal from a judgment only where the disposition either exceeds the maximum allowable by law or is unconstitutionally cruel or unusual. ORS 138.222 further provides that:
“(1) Notwithstanding the provisions of ORS * * * 138.050, a sentence imposed for a judgment of conviction entered for a felony committed on or after November 1, 1989, may be reviewed only as provided by this section.
«Hi * * * *
“(4) In any appeal, the appellate court may review a claim that:
“(a) The sentencing court failed to comply with requirements of law in imposing or failing to impose a sentence;
“(b) The sentencing court erred in ranking the crime seriousness classification of the current crime or in determining the appropriate classification of a prior conviction or juvenile adjudication for criminal history purposes; or
“(c) The sentencing court erred in failing to impose a minimum sentence that is prescribed by ORS 137.700 or 137.707.”
In this case, defendant pleaded no contest. Under ORS 138.050, therefore, his conviction is not reviewable. To the extent that defendant challenges only the sentence, his claim of error still is unreviewable. The crime occurred after November 1,1989. Our review of his appeal, therefore, is limited to the matters listed in ORS 138.222. Defendant appears to contend that the trial court’s error is that it “failed to comply with requirements of law,” as described in ORS *186138.222(4)(a). Not just any failure to comply with the law is reviewable on direct appeal, however. The statute refers to a failure to comply with the requirements of the law “in imposing or failing to impose a sentence.” The question, then, is whether failing to comply with the law “in imposing or failing to impose a sentence” includes failing to certify an interpreter. We conclude that it does not.
We begin with the text of the statute. It refers to a failure to comply with the law “in imposing” a sentence or in “failing to impose” a sentence. The language suggests that the focus of the statute is the sentence itself, not procedures that lead to the actual imposition of the sentence. To be sure, that significantly narrows the scope of matters reviewable on direct appeal. But that is precisely what the legislature intended. As the Supreme Court explained in State ex rel Huddleston v. Sawyer, 324 Or 597, 607, 932 P2d 1145, cert den _ US _, 118 S Ct 557, 139 L Ed 2d 399 (1997), “[t]he purpose of ORS 138.222, as revealed in the legislative history, was to curtail appellate review and reduce the number of appeals.”
Consistent with that purpose, the courts of this state have construed ORS 138.222 to apply only to the lawfulness of the sentence itself, not the procedures by which it was imposed. Relevant to the point is the Supreme Court’s decision in State v. Adams, 315 Or 359, 847 P2d 397 (1993), in which the court rejected the defendant’s contention that the failure of the trial court to make findings in support of a departure sentence was reviewable under ORS 138.222. Even more to the point is our decision in State v. Henderson, 116 Or App 604, 843 P2d 459 (1992), mod on other grounds 124 Or App 426, 861 P2d 406 (1993). In that case, the trial court sentenced defendant on a guilty plea under one statute. The court had second thoughts, vacated the original sentence and imposed a different — and longer — sentence under a different statute. Defendant appealed, challenging the authority of the court to vacate the original sentence. We held that the matter was not reviewable under ORS 138.222(4)(a). We explained:
“Under ORS 138.222(4)(a), in any appeal we may review a claim that the sentencing court failed to comply with the *187sentencing requirements. However, that subsection is subject to the general provision that a sentence may be reviewed under the provisions of ORS 138.222. ORS 138.222(1). Defendant’s argument challenges the vacation of the original sentence, not the sentence.”
Henderson, 116 Or App at 606 (emphasis in original).
It bears emphasis that, merely because a given matter is not subject to review on direct appeal does not mean that it is beyond appellate review. Other remedies remain available, depending on the particular challenge asserted by the defendant. For example, a challenge to the constitutionality of a sentence that is within the presumptive sentence prescribed by the felony sentencing guidelines is beyond the scope of direct appellate review, as provided in ORS 138.222(2)(a). The matter nevertheless may be subject to direct Supreme Court review in original mandamus. That was the case in Huddleston, in which the court held that, precisely because of the narrow scope of direct appellate review under ORS 138.222(4)(a), the state could proceed in mandamus to challenge the trial court’s decision as to the constitutionality of the defendant’s mandatory sentence. Huddleston, 324.Or at 608.
Similarly, post-conviction relief is available expressly for those cases in which a defendant wishes to assert a challenge to the constitutionality of a conviction that could not have been asserted on direct appeal. ORS 138.550(2). Thus, for example, a defendant who wishes to assert the unconstitutionality of a conviction on the ground of counsel’s failure to advise of the consequences of a guilty plea may do so in a post-conviction petition. See, e.g., Stelts v. State of Oregon, 299 Or 252, 701 P2d 1047 (1985) (failure to understand that consequences of guilty plea would include waiving confrontation rights); Castro v. Maass, 139 Or App 69, 73, 910 P2d 1156 (1996) (counsel failed to explain consequences of stipulated facts trial).
In light of the foregoing authority, it is clear that defendant’s challenge in this case is not reviewable on direct appeal. The failure to certify an interpreter is not an error of law “in imposing or failing to impose a sentence” any more than was a failure to make departure findings, as in Adams, *188or the asserted improper vacation of a sentence, as in Henderson. It concerns the procedure by which the sentence is imposed, not the sentence itself. As we plainly stated in Henderson, only the sentence itself is subject to direct appellate review under ORS 138.222(4)(a).
The concurrence insists that the failure to certify an interpreter is an error of law “in imposing or failing to impose a sentence.” Its reasoning, however, does not confront the language of the statute or the cases construing it as we have set forth above. Instead, the concurrence asserts, in syllogistic fashion that, (1) ORS 138.222 was intended to permit review of “illegal” sentences, which includes sentences imposed by improper processes, 160 Or App at 192; (2) a sentence imposed on a defendant who did not understand the nature of the proceedings would be an “illegal” sentence, 160 Or App at 192-93; (3) therefore, a challenge based on a defendant’s failure to understand the proceedings is reviewable under ORS 138.222,160 Or App at 194 n 2.
The syllogism, however, rests on a false premise, namely, that ORS 138.222 was intended to permit broadly a review of any “illegal” sentence, whatever the source of the illegality. The defendants in Adams and Henderson asserted that they were being subjected to an “illegal” sentence, and, in both cases, the court held that their claims were not subject to review on direct appeal under ORS 138.222. The language of ORS 138.222(4)(a) is narrower than the concurrence suggests, and we are not free to ignore that fact. ORS 174.020.
The concurrence emphasizes the importance of interpreter certification. The emphasis is entirely appropriate, for the ability of an accused to understand the nature of a criminal proceeding is essential to the fair disposition of criminal charges against him or her. But it simply does not follow that, because important constitutional rights may be implicated in the procedure by which a sentence is imposed, those rights must be vindicated on direct appeal under ORS 138.222(4)(a). Important constitutional rights also were *189implicated in Huddleston — including federal Equal Protection and Guaranty Clause rights, as well as state constitutional rights to equal privileges and immunities and of allo-cution — yet the court held that they were not subject to review on direct appeal. Id. at 608.
In that regard, it bears repeating that merely because a matter is not reviewable on direct appeal does not mean that the matter is not reviewable at all. A challenge to the constitutionality of a sentence not subject to direct appellate review under ORS 138.222, for example, remains subject to review, either in post-conviction or mandamus proceedings. Thus, the concurrence’s accusation that, by holding defendant’s challenge in this case unreviewable, we render the interpreter certification statutes “mostly meaningless in criminal cases,” 160 Or App at 194, is simply incorrect.
Defendant also contends that the trial court erred in imposing a unitary assessment and an award of attorney fees. Both matters may be resolved summarily. As to the unitary assessment, defendant contends that the trial court erred in imposing the fee without making a finding as to defendant’s ability to pay. The unitary assessment was imposed pursuant to ORS 137.290 (1995), which provided, in part:
“(1) In all cases of conviction for the commission of a crime * * * the trial court * * * shall impose upon the defendant, in addition to any other monetary obligation imposed, a unitary assessment under this section. * * * The unitary assessment is a penal obligation in the nature of a fine and shall be in an amount as follows:
“(a) $94 in the case of a felony.”
The statute further provides that the trial court may, in its discretion, waive the payment of the unitary assessment if, among other reasons, the court concludes that the fine will unduly burden the defendant in light of available financial resources and other obligations. ORS 137.290(3). The statute does not impose an affirmative obligation on the trial court to make any particular findings as a predicate to imposing the assessment in the first place.
*190As to the award of attorney fees, defendant entered no objection before the trial court and may not assert the error for the first time on appeal. State v. Daniel, 115 Or App 177, 178, 836 P2d 1366 (1992).
Affirmed.