diff --git a/src/lib/CoAlgFormula.ml b/src/lib/CoAlgFormula.ml
index 75d6f02b6d892fb151b0f4a1efb51cf4cd0a1d4e..5e20c9746910cb93abf3ddbecd89a7615f8acc8f 100644
--- a/src/lib/CoAlgFormula.ml
+++ b/src/lib/CoAlgFormula.ml
@@ -136,7 +136,7 @@ let rec iter func formula =
     func formula;
     let proc = iter func in (* proc = "process" *)
     match formula with
-    | TRUE | FALSE | AP _ -> ()
+    | TRUE | FALSE | AP _ | VAR _ -> ()
     | CONST _
     | CONSTN _ -> ()
     | NOT a | AT (_,a)
@@ -151,6 +151,7 @@ let rec iter func formula =
     | NORM(a, b) | NORMN(a, b) -> (proc a; proc b)
     | CHC (a,b)  -> (proc a ; proc b)
     | FUS (_,a) -> proc a
+    | MU (_, f) | NU (_, f) -> proc f
 
 let rec convert_post func formula = (* run over all subformulas in post order *)
   let c = convert_post func in (* some shorthand *)