diff --git a/theories/gmultiset.v b/theories/gmultiset.v
index 27db25b3cbbb6cbb26182d38bf0050f400d87733..2c9f85356f587d24d268df36167eb3b3e633214c 100644
--- a/theories/gmultiset.v
+++ b/theories/gmultiset.v
@@ -178,7 +178,7 @@ Proof.
destruct X as [X], Y as [Y]; unfold elements, gmultiset_elements.
set (f xn := let '(x, n) := xn in replicate (S n) x); simpl.
revert Y; induction X as [|x n X HX IH] using map_ind; intros Y.
- { by rewrite (left_id_L _ _), map_to_list_empty. }
+ { by rewrite (left_id_L _ _ Y), map_to_list_empty. }
destruct (Y !! x) as [n'|] eqn:HY.
- rewrite <-(insert_id Y x n'), <-(insert_delete Y) by done.
erewrite <-insert_union_with by done.