15-month-olds behave as if they comprehend filler-gap dependencies such as "wh"-questions and relative clauses. On one hypothesis, this success does not reflect adult-like representations but rather a "gap-driven" interpretation heuristic based on verb knowledge. Infants who know that "feed" is transitive may notice that a predicted direct object is missing in "Which monkey did the frog feed" __? and then search the display for the animal that got fed. This gap-driven account predicts that 15-month-olds will perform accurately only if they know enough verbs to deploy this interpretation heuristic; therefore, performance should depend on vocabulary. We test this prediction in a preferential looking task and find corroborating evidence: Only 15-month-olds with higher vocabulary behave as if they comprehend wh-questions and relative clauses. This result reproduces the previous finding that 15-month-olds can identify the right answer for wh-questions and relative clauses under certain experimental contexts, and is moreover consistent with the gap-driven heuristic account for this behavior.