diff --git a/iris_heap_lang/proofmode.v b/iris_heap_lang/proofmode.v
index 2d78d4319c941f329510af9964d614001fd25e5a..9d44d43ee5565ca11851c1eac9a820a3859a70b0 100644
--- a/iris_heap_lang/proofmode.v
+++ b/iris_heap_lang/proofmode.v
@@ -473,7 +473,7 @@ Lemma tac_twp_cmpxchg_fail Δ s E i K l q v v1 v2 Φ :
 Proof.
   rewrite envs_entails_eq. intros. rewrite -twp_bind.
   eapply wand_apply; first exact: twp_cmpxchg_fail.
-  rewrite envs_lookup_split //=. by do 2 f_equiv.
+  rewrite envs_lookup_split //=. simpl. by do 2 f_equiv.
 Qed.
 
 Lemma tac_wp_cmpxchg_suc Δ Δ' s E i K l v v1 v2 Φ :