diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index 12291a7..0e14525 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -83,6 +83,11 @@
 	 * 64 bit parent inode number.
 	 */
 	FILEID_NILFS_WITH_PARENT = 0x62,
+
+	/*
+	 * Filesystems must not use 0xff file ID.
+	 */
+	FILEID_INVALID = 0xff,
 };
 
 struct fid {