diff --git a/libraries/libmdb/mdb.c b/libraries/libmdb/mdb.c
index 935d37d694..f9831bb135 100644
--- a/libraries/libmdb/mdb.c
+++ b/libraries/libmdb/mdb.c
@@ -42,7 +42,7 @@
 #define MDB_VERSION	 1
 #define MAXKEYSIZE	 255
 
-#define P_INVALID	 (~0L)
+#define P_INVALID	 (~0UL)
 
 #define F_ISSET(w, f)	 (((w) & (f)) == (f))