OPINION
HOLCOMB J.,delivered the opinion of the Court,
in which MEYERS, PRICE, WOMACK, and COCHRAN, JJ„ joined.Appellant was convicted of possession of a controlled substance with intent to deliver. Punishment was assessed at imprisonment for twenty years and a $2,000 fíne. Appeal was taken to the Sixth Court of Appeals. There, appellant complained that the trial court erred in excluding testimony that the drugs recovered belonged to someone else and that the exclusion of that evidence was harmful. With one justice dissenting, the court of appeals held that the trial court erred in excluding the evidence but also held that appellant was not harmed by the error. Ray v. State, 148 S.W.3d 218 (Tex.App.-Texarkana *8352004). We granted review to determine whether the court of appeals erred in its harm analysis.
The Relevant Facts
Police stopped and searched a car in which appellant was the front-seat passenger. A medicine bottle containing eight rocks of crack cocaine was recovered from a narrow space between the passenger seat and the center console. Police also recovered narcotics paraphernalia from appellant’s purse. At trial, appellant admitted to possessing the paraphernalia but challenged the State’s theory that she had care, custody, and control of the drugs. Appellant’s testimony denying possession of the drugs was received by the trial court, but the trial court excluded the testimony of Owen Williams, an acquaintance of appellant, who had been with both appellant and the driver of the car shortly before the car was stopped. By offer of proof, appellant showed that Williams would have testified that the driver of the car took one rock of crack cocaine from a container, similar to the one found in the car, and gave it to Williams just before appellant and the driver of the car were stopped by police.
The court of appeals concluded correctly that Williams’s testimony should have been admitted because it had a tendency to make the existence of a fact of consequence more or less probable than it would have been without the evidence. Ray, 148 S.W.3d at 225 (citing Tex.R. Evid. 401). The court of appeals then considered whether appellant was harmed, first by applying and rejecting a constitutional error standard under Texas Rule of Appellate Procedure 42.2(a), and then by applying a non-constitutional error standard under 44.2(b).
Analysis and Holdings
Standard for constitutional error
Reasoning from our holding in Potier v. State, 68 S.W.3d 657, 665 (Tex.Crim.App.2002), the court of appeals concluded that the trial court’s error did not prevent appellant from presenting her defense, and therefore, the error was not of constitutional dimension. Ray, 148 S.W.3d at 226.
In Potier, this Court held that the exclusion of a defendant’s evidence can sometimes amount to a violation of his right to compel the attendance of witnesses in his favor. Potier v. State, 68 S.W.3d at 659; see also Washington v. Texas, 388 U.S. 14, 87 S.Ct. 1920, 18 L.Ed.2d 1019 (1967) (Sixth Amendment insures compulsory process to obtain favorable witnesses). However, “evidentiary rulings rarely rise to the level of denying the fundamental constitutional rights to present a meaningful defense.” Potier, 68 S.W.3d at 663. There are two circumstances in which the improper exclusion of evidence may establish a constitutional violation: (1) when a state evidentiary rule categorically and arbitrarily prohibits the defendant from offering relevant evidence that is vital to his defense; or (2) when a trial court erroneously excludes relevant evidence that is a vital portion of the case and the exclusion effectively precludes the defendant from presenting a defense. Id. at 659-62; Wiley v. State, 74 S.W.3d 399, 405 (Tex.Crim.App.2002).
Because the trial court’s exclusion of the evidence in this case was not grounded on any evidentiary rule prohibiting the admission of the testimony she offered, we are concerned only with the second category. Therefore, we must determine whether the exclusion of the evidence effectively prevented appellant from presenting a defense. Wiley, 74 S.W.3d at 405.
*836Through her own testimony, appellant was able to present to the jury her defense that the drugs did not belong to her and that, instead, the drugs belonged to the driver of the car. As this was the appellant’s own testimony, however, the jurors were likely to give it less weight than the testimony of Williams, an acquaintance who was willing to testify under oath that he had been given illegal drugs by the driver of the car, that the drugs he received were similar to those discovered by police, and that those drugs were found in a similar container. This testimony would have incrementally furthered appellant’s defensive theory. See, e.g., Cohn v. State, 849 S.W.2d 817, 820 (Tex.Crim.App.1993). Nevertheless, because appellant was permitted to testify about her defensive theory, we cannot say that the exclusion of Williams’s testimony effectively prevented her from presenting her defense.1 We thus conclude that the court of appeals did not err in declining to apply a constitutional harm analysis under Tex.R.App. P. 44.2(a). We now turn to whether the court of appeals correctly analyzed the error under Tex.R.App. P. 44.2(b).
Standard for non-constitutional error
When evaluating harm from non-constitutional error flowing from the exclusion of relevant evidence, we examine the record as a whole, and if we are fairly assured that the error did not influence the jury or had but a slight effect, we conclude that the error was harmless. Morales v. State, 32 S.W.3d 862, 867 (Tex.Crim.App.2000). In this case, we do not have fair assurance that the exclusion of Williams’s testimony did not influence the jury or had but a slight effect. See Johnson v. State, 967 S.W.2d 410 (Tex.Crim.App.1998).
A review of the record as a whole reveals that the question of possession was not only the most important issue in the case, it was the only contested issue in the case. Appellant was prejudiced because she was precluded from presenting third-party witness testimony which would have corroborated and given independent credibility to the defense she sought to establish. Because appellant’s only argument was that she did not possess the drugs, and the State’s case rested on a contrary argument, the erroneous exclusion of testimony that tended to establish possession in another was a “serious” error. See Ray, 148 S.W.3d at 227 (Carter, J., dissenting).
The court of appeals’s majority opinion reasons that the error was harmless because Williams’s testimony “would not have added significantly to Ray’s defense.” Id. at 226. However, as Justice Carter points out, this was an issue for the jury to decide—not the court of appeals. Id. at 227. As the jury did not have the benefit of the third-party testimony upon the most critical element the State had the burden to prove, we cannot say with fair assurance that the error did not influence the jury or had but a slight effect. Tex.R.App. P. 44.2(b). We reverse the judgment of the court of appeals and remand the cause to the trial court for further proceedings.
HERYEY, J., filed a concurring opinion, in which JOHNSON, KEASLER, and COCHRAN, JJ., joined. KELLER, P.J., filed a dissenting opinion.. Had appellant chosen not to testify in her own defense, we would be faced with exclusion of clearly vital evidence, precluding altogether the presentation of the defensive theory.