diff --git a/include/geos/gconst.h b/include/geos/gconst.h
index 55a99a21f..a1b6a1d86 100644
--- a/include/geos/gconst.h
+++ b/include/geos/gconst.h
@@ -9,7 +9,9 @@
 #ifndef _GCONST_H
 #define _GCONST_H
 
+#ifndef NULL
 #define NULL            ((void *) 0)
+#endif
 #define TRUE            0xff
 #define FALSE           ~TRUE
 #define MOUSE_SPRNUM    0