In most standard dialects of English, there is a gap in the paradigm of the verb BE where we expect to find amn't. But how do we know that this gap exists, since learners have no positive evidence that amn't is ungrammatical? It is even more puzzling since there is no gap when the subject is inverted (aren't I . . . ?). Familiar explanations for this gap fail; in particular, it cannot be the result of conservative acquisition strategies. The explanation offered here is based crucially on a combination of multiple-default inheritance and function-based morphology, as embodied in word grammar. The gap is due to a Nixon-diamond conflict between two competing values for the same morphological function required by the categories negative and first-person. The inverted form is supplied by stipulation because of the functional pressure to supply a 'casual' form. Various dialect alternatives to the Standard English pattern are also considered. The success of this explanation shows that language must use default inheritance, multiple nonorthogonal inheritance, and morphological functions.