diff --git a/lib/minix/include/string.h b/lib/minix/include/string.h
index 91a3834c..caff01c6 100644
--- a/lib/minix/include/string.h
+++ b/lib/minix/include/string.h
@@ -5,7 +5,9 @@
 #ifndef _STRING_H
 #define _STRING_H
 
+#ifndef NULL
 #define NULL    ((void *)0)
+#endif
 
 #ifndef _SIZE_T
 #define _SIZE_T