diff --git a/diff.h b/diff.h
index 95ed7f8..6a71013 100644
--- a/diff.h
+++ b/diff.h
@@ -70,6 +70,7 @@
 #define DIFF_OPT_DIFF_FROM_CONTENTS  (1 << 22)
 #define DIFF_OPT_SUBMODULE_LOG       (1 << 23)
 #define DIFF_OPT_DIRTY_SUBMODULES    (1 << 24)
+#define DIFF_OPT_IGNORE_UNTRACKED_IN_SUBMODULES (1 << 25)
 
 #define DIFF_OPT_TST(opts, flag)    ((opts)->flags & DIFF_OPT_##flag)
 #define DIFF_OPT_SET(opts, flag)    ((opts)->flags |= DIFF_OPT_##flag)