diff --git a/sys/dev/disk/dm/dm_pdev.c b/sys/dev/disk/dm/dm_pdev.c
--- a/sys/dev/disk/dm/dm_pdev.c
+++ b/sys/dev/disk/dm/dm_pdev.c
@@ -150,6 +150,16 @@
 	}
 	dmp->ref_cnt = 1;
 
+	/* Reject non-device vnodes (e.g. regular files) which have no
+	 * v_rdev.  Without this, dev_dioctl(NULL, DIOCGPART, ...) below
+	 * dereferences v_rdev and panics. */
+	if (dmp->pdev_vnode->v_rdev == NULL) {
+		dmdebug("%s is not a block device\n", dev_name);
+		dm_pdev_free(dmp);
+		lockmgr(&dm_pdev_mutex, LK_RELEASE);
+		return NULL;
+	}
+
 	if (dm_pdev_get_vattr(dmp, &va) == -1) {
 		dmdebug("getattr on %s failed\n", dev_name);
 		dm_pdev_free(dmp);