Defendant appeals from convictions for unlawful sexual penetration, ORS 163.411, and sexual abuse in the first degree, ORS 163.427. We reverse.
At the time of the incident giving rise to this prosecution, the alleged victim, K, was 14 years, 9 months old. Defendant was 16. Along with several friends, they were spending the evening in the Weston business district when defendant reported that a large party was taking place in the nearby mountains. K and two friends, one of whom was defendant’s sister, decided to drive up to the party. Defendant felt sick and went home.
When defendant’s mother learned that defendant’s sister was going to the party in the mountains in a car “borrowed” from a friend without his permission and driven by an underage driver, she called defendant’s sister and demanded that she return home. The girls returned to defendant’s house. K decided to spend the night with defendant’s sister. Eventually, the youths retired to the TV room where they fell asleep. Defendant and K were both on a couch, sleeping head-to-toe under a blanket. Early in the morning, K awoke. Defendant was sitting up and his “fingers were in her vagina.” When asked at trial how she responded, K explained,
“I flung my hand like this, so he thought I was still sleeping, and I closed my legs, and so I figured he would get the hint and, like, a few seconds later, he tried to open my legs, and he realized I wasn’t going to move my legs, so he laid back down. And a few seconds later, I was dumbfounded, and so I jumped off the side of the couch and went into the bathroom and I didn’t know what to do. I mean, he had been a friend, almost like a brother to me. So I went upstairs, and I didn’t know what to do so I went back downstairs and got my stuff, got dressed, came back upstairs, and it was really early to call anybody, because it would be out of the ordinary for me, but I couldn’t take it anymore. I couldn’t be in that house so I called [a friend] probably about 6:45 or so.”
Defendant’s version of events differed from K’s. According to defendant, he and K were lying under a blanket, *169sleeping, when she woke him up by rubbing her toes against him. Defendant testified that he responded by rubbing K’s feet and legs and that she never objected to his advances, leading him to believe she was awake and consenting. The trial court, acting as factfinder, believed K and not defendant; we therefore take her version, and not his, as true. Or Const, Art VII (Amended), § 3. In any event, he admitted eventually penetrating her vagina with his fingers.
After K reported the events, defendant was arrested and charged by a grand jury as follows:
“COUNT 1
“The defendant, on or about 08/11/01 * * * did unlawfully and knowingly penetrate the vagina of [K], with an object, to-wit: his finger, said [K] being incapable of consent by reason of physical helplessness, and
“COUNT 2
“on or about 08/11/01 * * * did unlawfully and knowingly subject [K], a person who was physically helpless, to sexual contact, by touching her vagina, a sexual and intimate part of [K].”
After a trial to the court, he was convicted on both counts and sentenced to a 100-month term for count 1 and a 75-month term for count two, the terms to run concurrently.
Defendant’s first two assignments of error concern the admission at trial of evidence of an incident that occurred when defendant was nine years old. On cross-examination by defense counsel, defendant’s mother testified that she was surprised about the allegations against defendant because
“[defendant] doesn’t even have girlfriends. You know, maybe he takes them to proms, but mostly, he doesn’t go out with girls. I mean, not that he’s strange, but he just, you know, was waiting for the right girl in his life, and that’s what he told me.”
Defense counsel then asked, “So is it fair then to say that he’s not sexually aggressive?” Defendant’s mother answered, ‘Very fair.”
*170On re-examination by the prosecutor, the following dialogue occurred,
“Q. What’s your basis of knowledge that he’s not sexually aggressive?
“A. My basis of knowledge? Well, he’s my son.
“Q. So you know about past events he’s been involved with?
“A. Yes. My children are very open with me about things that go on.
“Q. He’s had some prior circumstance of inappropriate behavior?
“A. No.
“[Defense counsel]: Your Honor, I’m going to object.
“[Prosecutor]: He opened the door, Your Honor.
“ [Defense counsel]: For three—
“[Prosecutor]: By asking her opinion of him, whether sexually aggressive or not.”
The trial court indicated that it would hear the evidence before making a ruling. The witness then explained, “The only thing that I know is there was an incident where they had a fort, and in order for other people to go in, they would have to look at his penis.” The witness testified further that she was unaware of any claim that “there was touching involved.” Ultimately, the trial court ruled, “I’ll admit [the evidence] for the extremely limited basis of the credibility of the witness. I am not admitting it for substantive evidence.” In other words, the testimony did not serve to prove that the fort incident did, in fact, occur, or to prove that defendant was, in fact, sexually aggressive. Rather, it served to prove that defendant’s mother was not a credible witness. The trial court’s ruling is the subject of defendant’s first assignment of error on appeal.
The testimony by defendant’s mother that defendant was not sexually aggressive is evidence of a character trait offered to show that it would have been out of character for defendant to have touched K without her consent. OEC 404(2) authorizes the admission of such evidence:
*171“Evidence of a person’s character is not admissible for the purpose of proving that the person acted in conformity therewith on a particular occasion, except:
“(a) Evidence of a pertinent trait of character offered by an accused, or by the prosecution to rebut the same.”
In turn, however, OEC 405 authorizes inquiry on cross-examination “into relevant specific instances of conduct” for purposes of impeachment.1 That is what occurred here. Defendant sought to offer character evidence through defendant’s mother that he was not sexually aggressive. Whether evidence is relevant presents a question of law. State v. Davis, 336 Or 19, 25, 77 P3d 1111 (2003). Although the question is close, we conclude that the fact that defendant’s mother knew about the fort episode — an episode that the only expert to appear at trial called “normal” — had no bearing on whether she lacked credibility when she asserted that, at the age of 16, defendant was not sexually aggressive. The evidence was not relevant to her credibility.
However, the error in allowing the prosecutor to elicit the testimony from mother was not prejudicial. Error is not prejudicial if we conclude that there is little likelihood that the error affected the court’s verdict. State v. Isom, 306 Or 587, 595-96, 761 P2d 524 (1988). Put another way, error is not prejudicial unless it “could have affected the [factfinder’s] determination.” Davis, 336 Or at 35. In determining whether defendant was or was not sexually aggressive, the factfinder, we are convinced, would not have been influenced by evidence that his mother’s opinion to the contrary was less than credible.
We reach a different conclusion, however, on defendant’s second assignment of error, which deals with different testimony about the fort incident. That testimony was not only erroneously admitted, it was also prejudicial.
*172 The fort incident arose a second time during cross-examination of defendant. The prosecutor asked defendant if his contact with K was a “mistake.” The following dialogue ensued:
“A. I guess so because it’s brought me all the way to here, and it’s not where I wanted to be with that kind of contact.
“Q. Now, you should know better than to make those kind of mistakes, shouldn’t you?
“A. Not really, because I’m new at this and I didn’t know.
“Q. Didn’t you have a problem when you were nine?”
At that point, defense counsel objected under OEC 404(3) on the grounds that the evidence was not probative and that its prejudicial effect outweighed its probative value.2 The trial court overruled defendant’s objection, and the prosecutor proceeded to examine defendant about the details of the fort incident.
On appeal, defendant asserts:
“One of the Defense theories was that Defendant mistakenly believed that [K] was awake when he inserted his fingers into her vagina. The prosecutor elicited testimony from Defendant about the ‘fort’ to rebut Defendant’s claim of mistake. Evidence that Defendant exposed his penis at a ‘fort’ when he was nine years old was not relevant to the issue of whether he mistakenly believed that [K] was awake, and the court should have excluded it.”
As indicated above, defendant based his case on his assertion that he did not know that K was asleep and incapable of consenting. Under ORS 163.427, defendant had the burden of proving that he mistakenly believed that K was awake, and, of course, the state was entitled to rebut defendant’s evidence in that regard. OEC 404(3) authorizes the admission of evidence of other crimes, wrongs, or acts to show an “absence of *173mistake.” The state offered the fort incident evidence to rebut defendant’s claim that he mistakenly thought that K was awake when he touched her. The proponent of the admission of evidence that is offered to show an absence of mistake must first demonstrate that the evidence is logically relevant. State v. Wert, 144 Or App 581, 584, 927 P2d 1103 (1996), rev den, 325 Or 369 (1997).
Under OEC 401, evidence is relevant if it has a “tendency to make the existence of any fact that is of consequence to the determination of the action more probable or less probable than it would be without the evidence.” The state argues that the evidence of the fort incident was evidence of “defendant’s thought processes” and “was relevant to prove the state’s theory that this was a situation in which defendant ignored any signals that would have suggested the impropriety of his conduct and forged ahead solely upon the basis of opportunity.” We are unpersuaded by the state’s argument. There is no similarity between the uncharged and charged misconduct, and they are separated in time by seven years. It is not apparent to us why the evidence about the fort incident affects the probability that defendant was acting under the mistaken belief that K was awake. We conclude that the trial court erred in overruling defendant’s objection to the state’s cross-examination of defendant on that subject.
Further, the error was not harmless. If erroneously admitted evidence relates to a “central factual issue” to the case, it is more likely to have affected the determination than if it dealt with a tangential issue. State v. Marrington, 335 Or 555, 566, 73 P3d 911 (2003). Furthermore, “the less substantial the evidence of guilt, the more likely it is that an error affected the result!.]” State v. Hansen, 304 Or 169, 180, 743 P2d 157 (1987). In the present case, the erroneously admitted evidence went directly to the central dispositive disputed issue, and it did so in a case where the relevant and admissible evidence on that issue was extremely close.
As explained above, the court allowed the prosecutor to elicit testimony from defendant about the fort incident because the court believed that the testimony tended to show that defendant knew K was asleep when he digitally penetrated her. Had defendant been able to establish that he *174thought that the victim was awake, he would have been acquitted. That is so because defendant was convicted of violating ORS 163.411 and ORS 163.427. The former provides:
“(1) [A] person commits the crime of unlawful sexual penetration in the first degree if the person penetrates the vagina, anus or penis of another with any object other than the penis or mouth of the actor and:
“(a) The victim is subjected to forcible compulsion;
“(b) The victim is under 12 years of age; or
“(c) The victim is incapable of consent by reason of mental defect, mental incapacitation or physical helplessness.”
ORS 163.427 provides:
“(1) A person commits the crime of sexual abuse in the first degree when that person:
“(a) Subjects another person to sexual contact and:
“(A) The victim is less than 14 years of age;
“(B) The victim is subjected to forcible compulsion by the actor; or
“(C) The victim is incapable of consent by reason of being mentally defective, mentally incapacitated or physically helpless [.]”
The charging instrument specified that defendant was accused of violating both of these statutes based not on compulsion or age but on the victim’s “physical helplessness.” As defined in ORS 163.305, that term means “that a person is unconscious * * It is therefore undisputed that the state’s case depended on proving beyond a reasonable doubt that defendant penetrated the victim while she was unconscious, that is, while she was asleep.
Defendant, on the other hand, relied on ORS 163.325(3):
“In any prosecution under ORS 163.355 to 163.445 in which the victim’s lack of consent is based solely upon the incapacity of the victim to consent because the victim is mentally defective, mentally incapacitated or physically helpless, it is an affirmative defense for the defendant to *175prove that at the time of the alleged offense the defendant did not know of the facts or conditions responsible for the victim’s incapacity to consent.”
At trial, defendant admitted the act of digital penetration; the only disputed issue was whether or not he knew, when he committed the act, that K was asleep and therefore physically helpless. Thus, in erroneously allowing testimony on the ground that it showed defendant’s lack of mistake, the court allowed testimony that it believed relevant to the heart of defendant’s case and not to a tangential issue. When the evidence is in (or close to) equipoise, then even an otherwise insubstantial error may suffice to tip the scales one way or the other. Hansen, 304 Or at 180. In the present case, no persuasive competent evidence except for some lack of candor by defendant supported the inference that he knew K was awake. K did not testify that she spoke to defendant, and nobody else witnessed the interaction. Thus, regarding defendant’s affirmative defense of mistake, the case pit defendant’s testimony against the nonexistent inference that the trial court erroneously believed could be drawn from the fort incident.
In sum, the trial court erroneously admitted evidence on the ground that it was relevant to rebut defendant’s “mistake” defense. The state presented little or no other evidence that had that effect. In announcing its verdict, the trial court did not disclose what evidence it relied on. Compare State v. Hunter, 141 Or App 73, 918 P2d 104, rev den, 324 Or 78 (1996) (error found harmless because trial court specified that it did not rely on erroneously admitted evidence); State v. Shipp, 27 Or App 675, 557 P2d 244 (1976) (same). Under these circumstances, we cannot conclude that the error had little effect on the court’s verdict.3
Reversed and remanded for new trial.
OEC 405(1) provides:
“In all cases in which evidence of character or a trait of character of a person is admissible, proof may be made by testimony as to reputation or by testimony in the form of an opinion. On cross-examination, inquiry is allowable into relevant specific instances of conduct.”
OEC 404(3) provides that “[ejvidence of other crimes, wrongs or acts is not admissible to prove the character of a person in order to show that the person acted in conformity therewith. It may, however, be admissible for such other purposes, such as proof of motive, opportunity, intent, preparation, plan, knowledge, identity, or absence of mistake or accident.”
Defendant also argues that sentencing a 16-year-old first offender to over eight years of incarceration for an act that did not involve forcible compulsion and was committed against another teenager violates the Cruel and Unusual Punishment Clause of Article I, section 16, of the Oregon Constitution. That argument cannot prevail in light of this court’s decision in State v. Thorp, 166 Or App 564, 2 P3d 903 (2000), rev dismissed, 332 Or 559 (2001). That case was never reviewed by the Supreme Court.