This study investigates whether conceptualizable agents in the discourse play a role in English L2 overpassivization errors (nontargetlike passivization of unaccusative verbs, a subclass of intransitives). It hypothesizes that learners are more likely to make overpassivization errors in externally caused events (in which an agent or cause may form part of the speaker's mental representation) than in internally caused events (in which the cause or causer of the event is not clear). Advanced Chinese learners of English were asked to choose the more grammatical form (active or passive) in target sentences with unaccusative verbs. Each target sentence was embedded in two different contexts expressing external and internal causation. A significant difference in error rates was found between the two different contexts: Learners accepted passivized unaccusative verbs more frequently when an agent or cause was available than when it was not. This finding is taken as an indication that learners transitivize unaccusative verbs before they passivize them and that the degree of transitivization varies depending on the presence of conceptualizable agents in the discourse. Thus this paper argues against a purely syntactic analysis of interlanguage errors such as overpassivization and in favor of an approach that takes cognitive factors into account.