diff --git a/include/geos/gconst.h b/include/geos/gconst.h
index e70eb9304..55a99a21f 100644
--- a/include/geos/gconst.h
+++ b/include/geos/gconst.h
@@ -4,14 +4,14 @@
   reassembled by Maciej 'YTM/Elysium' Witkowiak
 */
 
-/* Here are constants which didn't fit into any other cathegory... */
+/* Here are constants which didn't fit into any other category... */
 
 #ifndef _GCONST_H
 #define _GCONST_H
 
-#define NULL            0
-#define FALSE           NULL
+#define NULL            ((void *) 0)
 #define TRUE            0xff
+#define FALSE           ~TRUE
 #define MOUSE_SPRNUM    0
 #define DISK_DRV_LGH    0x0d80