This study employed a paired priming paradigm to ask whether input features influence a child's propensity to use non-nominative versus nominative case in subject position, and to use non-nominative forms even when verbs are marked for agreement. Thirty English-speaking children (ages 2;6 to 3;7) heard sentences with pronouns that had non-contrasting case forms (e.g. Dad hugs "it" and "it" hugs Tigger) and it was hypothesized that these forms would lead to more errors (e.g. "Him" hugs Barney) in an elicited phrase more often than if the children heard contrasting case forms (e.g. Dad hugs "us" and "we" hug the doggie). Tense/agreement features were also examined in children's elicited productions. The findings were consistent with predictions, and supported the input ambiguity hypothesis of Pelham (2011). Implications for current accounts of the optional infinitive stage are discussed.