concurring in the judgment of the court.
I write separately because I believe Drapeau’s Sixth Amendment rights were violated when he was not permitted to expose Kristy Big Eagle’s bias in the form of her familial relationships on cross-examination. However, I concur in the judgment of the court because this violation was harmless error.
The Sixth Amendment guarantees a criminal defendant the right to confront and cross-examine the government’s witnesses. See Davis v. Alaska, 415 U.S. 308, 315-16, 94 S.Ct. 1105, 39 L.Ed.2d 347 (1974). In attacking a witness’ credibility, the criminal defendant may challenge the witness’ recollection of events or expose “possible biases, prejudices, or ulterior motives of the witness as they may relate directly to issues or personalities in the case at hand.” Id. at 316, 94 S.Ct. 1105. “Proof of bias is almost always relevant because the jury, as finder of fact and weigher of credibility, has historically been entitled to assess all evidence which might bear on the accuracy and truth of a witness’ testimony.” United States v. Abel, 469 U.S. 45, 52, 105 S.Ct. 465, 83 L.Ed.2d 450 (1984) (emphasis added). Accordingly, the criminal defendant’s Sixth Amendment right to confront and cross-examine his accuser includes the right to attack the credibility of a government witness by exposing to the jury facts that may establish bias. See Davis, 415 U.S. at 318, 94 S.Ct. 1105.
While the trial judge retains wide latitude to regulate cross-examination, all limitations placed on a criminal defendant’s Sixth Amendment right to confront government witnesses must pass constitutional muster. The Supreme Court defined the controlling test for constitutional error as follows:
*880[A] criminal defendant states a violation of the Confrontation Clause by showing that he was prohibited from engaging in otherwise appropriate cross-examination designed to show a prototypical form of bias on the part of the witness, and thereby “to expose to the jury the facts from which jurors ... could appropriately draw inferences relating to the reliability of the witness.”
Delaware v. Van Arsdall, 475 U.S. 673, 680, 106 S.Ct. 1431, 89 L.Ed.2d 674 (1986) (quoting Davis, 415 U.S. at 318, 94 S.Ct. 1105). A “key factor” in determining whether the limitation on cross-examination violated the defendant’s right of confrontation is whether the defendant was allowed other means to obtain the effect that the excluded examination would have achieved. United States v. Warfield, 97 F.3d 1014, 1024 (8th Cir.1996). “If a reasonable jury might have received a significantly different impression of a witness’ credibility had counsel been permitted to pursue the proposed line of cross-examination, defendant has stated a Confrontation Clause" violation.” United States v. Beckman, 222 F.3d 512, 524 (8th Cir.2000).
During Drapeau’s cross-examination of the government’s key witness he was prohibited from exposing that Big Eagle’s sister worked as a victim’s advocate in the same U.S. Attorney’s Office that was prosecuting Drapeau, and that this same sister’s husband (Big Eagle’s brother-in-law) was one of the FBI agents that arrested Drapeau. The government resisted Dra-peau’s offer of proof based on relevancy, arguing that Drapeau had already shown Big Eagle’s bias in the form of favorable treatment.
It’s not relevant to the matters in front of the Court to place in front of this jury for trial who the witnesses’ [sic] family members are .... This issue would only serve to confuse and mislead the jury into a collateral issue that’s not in front of the Court or isn’t at trial.
T. Tr. at 95. After concluding that neither Big Eagle’s sister nor brother-in-law had any authority at the U.S. Attorney’s Office to influence Big Eagle’s prosecution, the district court rejected Drapeau’s offer of proof.
[Hts not relevant and the offer of proof is rejected. It’s also wasting time under 403 and trying to confuse the jury. The issue is what happened out there that particular day. The motives of Kristy Big Eagle, to me, don’t mean anything anyway. Because, in fact, if that’s Mr. Drapeau’s voice on the tape, that’s all there is to it.
Id. at 96.
It was an error of law and therefore an abuse of discretion for the district court to conclude that the bias and motives of the government’s key witness did not mean anything. To the contrary, “the exposure of a witness’ motivation in testifying is a proper and important function of the constitutionally protected right of cross-examination.” Davis, 415 U.S. at 316-317, 94 S.Ct. 1105. Contrary to the district court’s conclusion, evidence of bias is not irrelevant, see id. at 317, 94 S.Ct. 1105, and contrary to the government’s position, “[potential bias is not a collateral issue.” Wealot v. Armontrout, 948 F.2d 497, 500 (8th Cir.1991); see also Johnson v. Brewer, 521 F.2d 556, 562 n. 13 (8th Cir.1975) (“In courtroom parlance, facts showing bias are not collateral.”) (internal quotation marks and citation omitted). Big Eagle’s familial relationships meant she was potentially biased and it was for the jury “as finder of fact and weigher of credibility” to consider evidence of this bias as it pondered Big Eagle’s testimony. Abel, 469 U.S. at 52, 105 S.Ct. 465. Family, bias was hardly a confusing concept for the jury to understand. Furthermore, allowing perhaps a few hours to expose and address this evidence would not have been a waste of time *881considering Drapeau faced several years in prison, and was in fact sentenced to serve twenty-seven years.
Additionally, exposure of Big Eagle’s family relationship to an FBI agent who arrested Drapeau would have bolstered Drapeau’s entrapment defense by showing that Big Eagle had motives to participate in the sting operation beyond the favorable treatment she received. It would have provided evidence that law enforcement had unique access to Big Eagle and significant influence over her. In an entrapment defense, the motivations of the government’s only cooperating witness can mean very much indeed. It was up to the jury to accept or reject this evidence.
It is admittedly rare that we reverse the district court for a Confrontation Clause violation. But when a criminal defendant is “prohibited from engaging in otherwise appropriate cross-examination designed to show a prototypical form of bias” to impeach the credibility of a critical government witness, constitutional error results. Van Arsdall, 475 U.S. at 680, 106 S.Ct. 1431; see also Beckman, 222 F.3d at 525 (finding violation of Confrontation Clause because limitation on cross-examination prevented defendant from establishing witness’ rebuffed sexual advances towards defendant’s wife gave witness a motivation to lie and established a distinct form of bias in addition to bias in the form of favorable treatment). The defendant’s opportunity to establish other forms of bias does not divest him of his constitutional right to expose relevant and non-eumulative witness bias. See id.
Our post-Eero Arsdall eases affirming the district court’s limitation of cross-examination, which are many, generally do so on the basis that the excluded information was cumulative evidence of the same form of bias or that it was irrelevant or collateral. This stands in contrast to the evidence Drapeau sought to expose about Big Eagle’s family relationships. The issue of Big Eagle’s family bias was relevant to Drapeau’s entrapment defense and it was the only evidence of this particular form of bias. Had the district court merely limited the depth of cross-examination on this point after allowing Drapeau to establish the basic facts of these relationships, I would have no problem deferring to the district court’s discretion in the matter. But a total exclusion of all evidence of this family bias leads me to conclude that “a reasonable jury might have received a significantly different impression” of Big Eagle’s credibility and her motivation to testify had they known her sister worked in the U.S. Attorney’s Office prosecuting Dra-peau, and that her brother-in-law was one of the officers who had arrested Drapeau. Beckman, 222 F.3d at 524. If I were a juror, I would want to know about this.
While I take issue with the majority’s holding that there was no Sixth Amendment violation, I agree the violation was harmless error. See Van Arsdall, 475 U.S. at 684, 106 S.Ct. 1431 (holding Confrontation Clause violations subject to harmless error analysis). Even without Big Eagle’s testimony, there was overwhelming evidence to support Drapeau’s conviction. Drapeau made incriminating statements regarding selling drugs to Big Eagle, there was an audio recording of the controlled drug buy, and. Big Eagle delivered the purchased methamphetamine to the agents who watched her enter and leave Drapeau’s house. Accordingly, I concur that the error was harmless in this case.