While early code-switching research (i.e., Poplack, 1980) focused on the possibility of universal constraints on switching, MacSwan’s (2010, 2014) “Constraint-Free” research program centers on the notion that code-switching is only constrained by the interaction of a bilingual’s two grammars. In following with this proposal, the current study examines whether two types of Spanish-English bilinguals are equally sensitive to the (un)grammaticality of Spanish-English code-switching at the subject-predicate and auxiliary-verb phrase boundaries. Twenty-five heritage Spanish speakers and forty-four L2 Spanish learners completed an Audio Naturalness Judgment Task in which they judged grammatical and ungrammatical Spanish-English code-switching at these two syntactic junctions. Results indicate that the L2 Spanish speakers and the heritage bilinguals, regardless of their self-reported exposure to code-switching, correctly differentiated between grammatical and ungrammatical switches, suggesting that they have implicit knowledge of code-switching grammaticality which falls out from syntactic knowledge of the two languages.