Abstract.  Some null-subject languages cannot drop the subject pronoun in the second conjunct in sentences which translate as They say that John doesn’t speak French, but he does, where the pronoun is coreferent with John. Among the null-subject languages which do not allow a null subject in this context are Chinese (various dialects), Finnish and European Portuguese. Among the languages that allow it are Japanese, Persian, and Spanish. An explanation is proposed of this variation, based on the following correlation: The languages which do not allow a null subject standardly reply to yes/no-questions by repeating the finite verb of the question. The languages which allow a null subject standardly reply by using a special affirmation particle ‘yes’. The connection between these two properties is that both involve polarity focus. The proposal is that a null subject in the second conjunct is prohibited in the former class because of a competition of derivations involving ellipsis. This hypothesis is based on the theory of polarity focus in Finnish articulated in Holmberg (2001).