diff --git a/lisp/dos-w32.el b/lisp/dos-w32.el
index 94ba5def7b5..aeebb9c3d08 100644
--- a/lisp/dos-w32.el
+++ b/lisp/dos-w32.el
@@ -74,6 +74,9 @@ against the file name, and TYPE is nil for text, t for binary.")
 	(setq alist (cdr alist)))
       found)))
 
+;; Silence compiler. Defined in src/buffer.c on DOS_NT.
+(defvar default-buffer-file-type)
+
 ;; Don't check for untranslated file systems here.
 (defun find-buffer-file-type (filename)
   (let ((match (find-buffer-file-type-match filename))