Recent research in the area of Second Language Acquisition has proposed that bilinguals and L2 learners show syntactic indeterminacy when syntactic properties interface with other cognitive domains. Most of the research in this area has focused on the pragmatic use of syntactic properties while the investigation of compliance with a grammatical rule at syntax-related interfaces has not received due attention. In this study, the compliance of 67 Persian native speakers and 52 Persian speaking L2 learners of English with the Overt Pronoun Constraint (OPC, henceforth) a proposed UG principle, at the syntax-pragmatics interface is investigated. Both groups of participants demonstrated violations of the OPC at the syntax-pragmatics interface. It is argued that the results of this study both confirm and complement Sorace and Filiaci’s (2006) Interface Hypothesis while showing that difficulties at interface contexts are more a result of interface complexities than cross-linguistic influence.