diff --git a/testing/scripts/build-guestimages b/testing/scripts/build-guestimages
index b93c868d2..f7fb1f85c 100755
--- a/testing/scripts/build-guestimages
+++ b/testing/scripts/build-guestimages
@@ -65,7 +65,7 @@ do
 		execute_chroot "update-rc.d bind9 defaults" 0
 	fi
 	sync
-	execute "umount $LOOPDIR" 0
+	execute "umount -l $LOOPDIR" 0
 	execute "qemu-nbd -d $NBDEV" 0
 	log_status 0
 done