diff --git a/src/plugins/wp/Conditions.ml b/src/plugins/wp/Conditions.ml
index 6f74f1dcde905233dcfc5a0bc4817c3e1a54d1db..51be9bba036ec3825f0fefb6b82f905379c9345a 100644
--- a/src/plugins/wp/Conditions.ml
+++ b/src/plugins/wp/Conditions.ml
@@ -1056,7 +1056,7 @@ struct
     | Some m -> m
     | None ->
         let m = Lang.sigma () in
-        F.Subst.add_map m s.def ;
+        F.Subst.add_fun m (fun e -> Tmap.find e s.def) ;
         s.cache <- Some m ; m
 
   let e_apply s e = F.e_subst (subst s) e