diff --git a/libraries/liblmdb/midl.c b/libraries/liblmdb/midl.c
index 8de61e3..7289f40 100644
--- a/libraries/liblmdb/midl.c
+++ b/libraries/liblmdb/midl.c
@@ -132,7 +132,7 @@ void mdb_midl_free(MDB_IDL ids)
 int mdb_midl_shrink( MDB_IDL *idp )
 {
 	MDB_IDL ids = *idp;
-	if (ids[-1] > MDB_IDL_UM_MAX) {
+	if (*(--ids) > MDB_IDL_UM_MAX) {
 		ids = realloc(ids, (MDB_IDL_UM_MAX+1) * sizeof(MDB_ID));
 		*ids++ = MDB_IDL_UM_MAX;
 		*idp = ids;