diff --git a/lisp/vc/ediff.el b/lisp/vc/ediff.el
index be4ced9b55f..a4244c941d2 100644
--- a/lisp/vc/ediff.el
+++ b/lisp/vc/ediff.el
@@ -1367,8 +1367,8 @@ buffer. If odd -- assume it is in a file."
     (require 'ediff-ptch)
     (setq patch-buf
 	  (ediff-get-patch-buffer
-	   (if arg (prefix-numeric-value arg))
-           (get-buffer patch-buf)))
+	   (and arg (prefix-numeric-value arg))
+           (and patch-buf (get-buffer patch-buf))))
     (setq source-dir (cond (ediff-use-last-dir ediff-last-dir-patch)
 			   ((and (not ediff-patch-default-directory)
 				 (buffer-file-name patch-buf))