diff --git a/src/lib/CoAlgMisc.ml b/src/lib/CoAlgMisc.ml
index 8fd1f68135254e104f961b5a58680c8df63ef81e..85d08bcabeaf8f13ceba5442f5eaa76fa0ac5440 100644
--- a/src/lib/CoAlgMisc.ml
+++ b/src/lib/CoAlgMisc.ml
@@ -1017,10 +1017,15 @@ let rec detClosure vars nomTbl hcF fset vset atset nomset s f =
        FL(μ X . φ) = {μ X . φ} ∪ FL(φ[X|->μ X . φ])
      *)
     | C.HCMU (name, f1) ->
-       let () = C.HcFHt.replace vset.(s) f name in
-       let unfold = C.hc_replace hcF name f f1 in
-       let appendvars = List.append newvars [name] in
-       detClosure appendvars nomTbl hcF fset vset atset nomset s unfold
+       begin
+         if C.hc_freeIn name f1
+         then
+           C.HcFHt.replace vset.(s) f name
+         else ();
+         let unfold = C.hc_replace hcF name f f1 in
+         let appendvars = List.append newvars [name] in
+         detClosure appendvars nomTbl hcF fset vset atset nomset s unfold
+       end
     | C.HCNU (name, f1) ->
        let unfold = C.hc_replace hcF name f f1 in
        detClosure_ nomTbl hcF fset vset atset nomset s unfold