diff --git a/lisp/textmodes/tex-mode.el b/lisp/textmodes/tex-mode.el
index 86a4b1d006e..d6be9d511d6 100644
--- a/lisp/textmodes/tex-mode.el
+++ b/lisp/textmodes/tex-mode.el
@@ -89,6 +89,7 @@ The command `tex-file' runs TeX on the file specified by `tex-main-file'
 if the variable is non-nil."
   :type '(choice (const :tag "None" nil)
                  file)
+  :safe (lambda (x) (or (stringp x) (null x)))
   :group 'tex-file)
 
 ;;;###autoload