The JACC subject policy context handler always returns null

Inside of a JACC policy provider, when the implies method is called, I attempt to obtain the caller's subject by invoking PolicyContext.getContext("javax.security.auth.Subject.container"). According to section 4.6.1.1 of the JACC specification, this should always return a non null subject, unless the caller hasn't been authenticated yet. When running against 4.0.2RC1, this always returns null.