diff --git a/include/basic_types.h b/include/basic_types.h
index 6e02b3d03..786c3a284 100644
--- a/include/basic_types.h
+++ b/include/basic_types.h
@@ -61,6 +61,9 @@ typedef  Word                   OffT;     // 32             64
 /* This is going to be either 4 or 8. */
 #define VG_WORDSIZE VEX_HOST_WORDSIZE
 
+#if !defined(NULL)
+#  define NULL ((void*)0)
+#endif
 
 /* ---------------------------------------------------------------------
    non-builtin types
diff --git a/include/tool.h b/include/tool.h
index 956048298..be561bb61 100644
--- a/include/tool.h
+++ b/include/tool.h
@@ -104,10 +104,6 @@ extern Addr VG_(get_IP) ( ThreadId tid );
 /*=== Valgrind's version of libc                                   ===*/
 /*====================================================================*/
 
-#if !defined(NULL)
-#  define NULL ((void*)0)
-#endif
-
 /* ------------------------------------------------------------------ */
 /* stdlib.h */