This paper examines the proposal that syntactic features not instantiated in L1 are unavailable to L2 learners, while semantic ones are available and used in a compensatory way (Tsimpli & Dimitrakopoulou 2007). On this assumption, the (over)use of null subjects in L2 English will be not random, but constrained by any relevant semantic features, as the study of Prentza & Tsimpli (in press) on advanced L2 grammars has suggested. Null subject availability distinguishes between Greek and English and follows from a difference in syntactic features. The semantic feature targeted here is referentiality of the subject, i.e. whether this is referential or expletive. Aiming to test the performance of very advanced learners, a judgement task was administered to 37 advanced and 12 very advanced learners, as well as to 25 English natives. The results showed that the very advanced group accepted significantly more null subjects than the controls. Crucially, omission was significantly higher when the subjects were expletive than referential. Thus, in very advanced grammars as well i) syntactic features are inaccessible leading to target-deviant performance ii) the semantic feature of Referentiality is accessible and has a compensatory role regulating L2 null subject use and eliminating real optionality from the system.