diff --git a/alloca.c b/alloca.c
index 12c11229..31fb4e0d 100644
--- a/alloca.c
+++ b/alloca.c
@@ -73,7 +73,9 @@ typedef void *pointer;
 typedef char *pointer;
 #endif
 
+#ifndef NULL
 #define	NULL	0
+#endif
 
 /* Different portions of Emacs need to call different versions of
    malloc.  The Emacs executable needs alloca to call xmalloc, because