Progress gauge crash

I've tried to replace (sit-for 0.5) with (sit-for 0), and encountered a crash. Lisp backtrace: redisplay-frame() # bind (inhibit-read-only zmacs-region-stays val frame value message) raw-append-progress("Fontifying *followup to Kai Großjohann on gnu.emacs.gnus*..." 0 #<x-frame "Gnus" 0x64f7>) (if (eq label (car top)) (progn (setcdr top message) (if ... ... ...) (redisplay-gutter-area)) (push (cons label message) progress-stack) (raw-append-progress message value frame)) ) # bind (tmsg top) (let* ((top ...) (tmsg ...)) (if (eq label ...) (progn ... ... ...) (push ... progress-stack) (raw-append-progress message value frame)) (dispatch-non-command-events) (when (input-pending-p) (dispatch-event ...)) (when (eq value 100) (sit-for 0 nil) (clear-progress label))) ) # bind (frame value message label) append-progress(font-lock "Fontifying *followup to Kai Großjohann on gnu.emacs.gnus*..." 0 nil) # bind (frame value message label) display-progress(font-lock "Fontifying *followup to Kai Großjohann on gnu.emacs.gnus*..." 0) # bind (str args value fmt label) lprogress(font-lock "Fontifying %s..." 0 "*followup to Kai Großjohann on gnu.emacs.gnus*") # bind (maybe-loudly end beg) font-lock-default-unfontify-region(1 1 t) # bind (loudly end beg) font-lock-unfontify-region(1 1 t) # bind (was-on font-lock-verbose font-lock-message-threshold aborted) font-lock-default-fontify-buffer() # bind (font-lock-verbose) font-lock-fontify-buffer() # bind (on-p maximum-size arg) font-lock-mode(1) turn-on-font-lock() # bind (explicit-defaults) font-lock-set-defaults() message-xmas-maybe-fontify() run-hooks(text-mode-hook-identify message-xmas-maybe-fontify) message-mode() ... C backtrace: