diff --git a/lib/idr.c b/lib/idr.c
index d0f1acd..09cbe2b 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -120,7 +120,7 @@
 	int n, m, sh;
 	struct idr_layer *p, *new;
 	int l, id, oid;
-	long bm;
+	unsigned long bm;
 
 	id = *starting_id;
  restart: