A fictionalist attitude towards an area of discourse encourages us to assent to certain sentences of that discourse without believing that they are true. Prima facie, this amounts to a suggestion that we should also assent to sentences of the form 'S but I don't believe that S'. Traditional versions of fictionalism have an answer to this challenge, but I argue that the answer is unavailable for a currently popular type of fictionalism. This is bad news for fictionalism in general (...) because the currently popular variety is the one that deals best with the Quine-Putnam indispensability argument. (shrink)