diff --git a/lib/idr.c b/lib/idr.c
index 1cac726c44bc17cc6fa1a7b5ab6358279a0c9a5d..0dc782216d4b94dc423c2d62d1ac70a2b6d30ddc 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -156,10 +156,12 @@ static int sub_alloc(struct idr *idp, int *starting_id, struct idr_layer **pa)
 			id = (id | ((1 << (IDR_BITS * l)) - 1)) + 1;
 
 			/* if already at the top layer, we need to grow */
-			if (!(p = pa[l])) {
+			if (id >= 1 << (idp->layers * IDR_BITS)) {
 				*starting_id = id;
 				return IDR_NEED_TO_GROW;
 			}
+			p = pa[l];
+			BUG_ON(!p);
 
 			/* If we need to go up one layer, continue the
 			 * loop; otherwise, restart from the top.