Focusing on the distribution of a/some and a certain indefinites in English, this paper argues for a unified in-situ mechanism for both kinds of indefinites. It proposes an account that separates the functional dependency between DPs from the semantics of determiners. While this idea is formulated within a choice functional treatment of indefinites, it can be incorporated into other in-situ accounts as well. This proposal avoids generating unattested readings (Binder Roof Constraint) while relying only on familiar and independently needed mechanisms. Thus, no new machinery is introduced to the syntax or semantics for the sole purpose of solving the overgeneration problem for the in-situ accounts of indefinites. Attributing the difference between the two kinds of indefinites with respect to the Binder Roof Constraint to the epistemic contribution of the NP determiner certain, this paper makes a novel typological prediction that indefinites that semantically encode the speaker's knowledge are not subject to the Binder Roof Constraint.