diff --git a/lisp/subr.el b/lisp/subr.el
index 3a60e15b0e5..2d6a72a6ac5 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -131,7 +131,7 @@ If N is bigger than the length of X, return X."
 	  (setq m (1+ m) p (cdr p)))
 	(if (<= n 0) p
 	  (if (< n m) (nthcdr (- m n) x) x)))
-    (while (cdr x)
+    (while (consp (cdr x))
       (setq x (cdr x)))
     x))