diff --git a/src/fns.c b/src/fns.c
index d1808440966..86ad333702e 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -1349,7 +1349,7 @@ The value is actually the tail of LIST whose car is ELT.  */)
   (register Lisp_Object elt, Lisp_Object list)
 {
   register Lisp_Object tail;
-  for (tail = list; !NILP (tail); tail = XCDR (tail))
+  for (tail = list; CONSP (tail); tail = XCDR (tail))
     {
       register Lisp_Object tem;
       CHECK_LIST_CONS (tail, list);
@@ -1397,7 +1397,7 @@ The value is actually the tail of LIST whose car is ELT.  */)
   if (!FLOATP (elt))
     return Fmemq (elt, list);
 
-  for (tail = list; !NILP (tail); tail = XCDR (tail))
+  for (tail = list; CONSP (tail); tail = XCDR (tail))
     {
       register Lisp_Object tem;
       CHECK_LIST_CONS (tail, list);
@@ -1710,7 +1710,7 @@ changing the value of a sequence `foo'.  */)
     {
       Lisp_Object tail, prev;
 
-      for (tail = seq, prev = Qnil; !NILP (tail); tail = XCDR (tail))
+      for (tail = seq, prev = Qnil; CONSP (tail); tail = XCDR (tail))
 	{
 	  CHECK_LIST_CONS (tail, seq);