diff --git a/sys/dev/disk/dm/dm_dev.c b/sys/dev/disk/dm/dm_dev.c
--- a/sys/dev/disk/dm/dm_dev.c
+++ b/sys/dev/disk/dm/dm_dev.c
@@ -192,7 +192,12 @@
 		TAILQ_INSERT_TAIL(&dm_dev_list, dev, next_devlist);
 		dm_dev_counter++;
 	} else {
-		KKASSERT(dmv != NULL);
+		/*
+		 * Device with this name or uuid already exists.
+		 * dmv may be NULL here (uuid was zero-filled, so the
+		 * uuid lookup was skipped, but the name/minor lookup
+		 * matched).  That is fine -- we return EEXIST either way.
+		 */
 		r = EEXIST;
 	}