diff --git a/include/string.h b/include/string.h
index 854359dad..abaf80e7d 100644
--- a/include/string.h
+++ b/include/string.h
@@ -37,9 +37,8 @@
 #define _STRING_H
 
 /* NULL pointer */
-#ifndef _HAVE_NULL
-#define NULL    0
-#define _HAVE_NULL
+#ifndef NULL
+#define NULL ((void *) 0)
 #endif
 
 /* size_t is needed */