diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index 1557978..b039a17 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -834,6 +834,15 @@
 	if (!(fl->fl_flags & FL_FLOCK))
 		return -ENOLCK;
 
+	/*
+	 * The NFSv4 protocol doesn't support LOCK_MAND, which is not part of
+	 * any standard. In principle we might be able to support LOCK_MAND
+	 * on NFSv2/3 since NLMv3/4 support DOS share modes, but for now the
+	 * NFS code is not set up for it.
+	 */
+	if (fl->fl_type & LOCK_MAND)
+		return -EINVAL;
+
 	if (NFS_SERVER(inode)->flags & NFS_MOUNT_LOCAL_FLOCK)
 		is_local = 1;