diff --git a/lisp.c b/lisp.c
index 99c1d33..6c792b7 100644
--- a/lisp.c
+++ b/lisp.c
@@ -241,7 +241,7 @@ void PrintList(int x) {
 #endif
   PrintChar('(');
   PrintObject(Car(x));
-  while ((x = Cdr(x))) {
+  while ((x = Cdr(x)) != NIL) {
     if (!ISATOM(x)) {
       PrintChar(' ');
       PrintObject(Car(x));