You’ll need to provide more information to be sure. Try setting
(setq debug-on-error t)
which will give you a stack trace showing what function is complaining about the string being nil.
My guess is that buffer-file-name is returning nil, and that’s where the problem lies (not all buffers have file names). Check out the debugging section of An Introduction To Programming in Emacs Lisp, or the debugging section of the Emacs Lisp manual.