diff --git a/src/plugins/wp/Letify.ml b/src/plugins/wp/Letify.ml
index 5d26dea8e0d2d894051787e19ceefe134df84ad1..8ed7a2287278690725bc73641ef396d1fc67fe57 100644
--- a/src/plugins/wp/Letify.ml
+++ b/src/plugins/wp/Letify.ml
@@ -271,7 +271,7 @@ struct
     | None ->
         let s = Lang.sigma () in
         F.Subst.add_fun s (lookup sigma.def) ;
-        F.Subst.add_map s sigma.cst ;
+        F.Subst.add_fun s (fun e -> Tmap.find e sigma.cst) ;
         F.Subst.add_filter s (filter sigma.dall) ;
         sigma.cache <- Some s ; s