diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9b87bae3ebb6ab058ef8118abdd0215732452927..601a57ecb94c3fe2fa7eee3b007793a4efb2be93 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -152,6 +152,7 @@ API-breaking change is listed.
   `map_filter_singleton_False`, `map_filter_comm`, `map_union_least`,
   `map_subseteq_difference_l`, `insert_difference`, `insert_difference'`,
   `difference_insert`, `difference_insert_subseteq`. (by Hai Dang)
+- Add `map_size_disj_union`.
 - Tweak reduction on `gset`, so that `cbn` matches the behavior by `simpl` and
   does not unfold `gset` operations. (by Paolo G. Giarrusso, BedRock Systems)
 - Add `get_head` tactic to determine the syntactic head of a function