diff --git a/sys/vfs/smbfs/smbfs_vnops.c b/sys/vfs/smbfs/smbfs_vnops.c
--- a/sys/vfs/smbfs/smbfs_vnops.c
+++ b/sys/vfs/smbfs/smbfs_vnops.c
@@ -805,6 +805,8 @@
 
 	if (bio->bio_flags & BIO_SYNC)
 		error = smbfs_doio(ap->a_vp, bio, proc0.p_ucred, td);
+	else
+		error = EOPNOTSUPP;
 	return error;
 }