diff --git a/sbin/mdconfig/mdconfig.c b/sbin/mdconfig/mdconfig.c
index ed781763f3d2..280219fcb5d6 100644
--- a/sbin/mdconfig/mdconfig.c
+++ b/sbin/mdconfig/mdconfig.c
@@ -213,6 +213,13 @@ main(int argc, char **argv)
 		case 'u':
 			if (cmdline != 2 && cmdline != 3)
 				usage();
+			if (!strncmp(optarg, "/dev/", 5))
+				optarg += 5;
+			if (!strncmp(optarg, MD_NAME, sizeof(MD_NAME) - 1))
+				optarg += sizeof(MD_NAME) - 1;
+			mdio.md_unit = strtoul(optarg, &p, 0);
+			if (mdio.md_unit == (unsigned)ULONG_MAX || *p != '\0')
+				errx(1, "bad unit: %s", optarg);
 			mdunit = optarg;
 			mdio.md_options &= ~MD_AUTOUNIT;
 			break;