--- sys/kern/vfs_syscalls.c.orig
+++ sys/kern/vfs_syscalls.c
@@ -409,6 +409,8 @@
 	case MNT_NOWAIT:
 		break;
 	default:
+		if (bufseg == UIO_SYSSPACE)
+			*buf = NULL;
 		return (EINVAL);
 	}
 restart: