diff --git a/src/lib/CoAlgFormula.ml b/src/lib/CoAlgFormula.ml
index 0dd1215441474a55875ac4a2c6ddbd900ab1159c..2245965b130230b944ba7bfd2cf54d5eb0a96166 100644
--- a/src/lib/CoAlgFormula.ml
+++ b/src/lib/CoAlgFormula.ml
@@ -254,7 +254,19 @@ let verifyMuMonotone forumla =
   true
 
 let verifyMuGuarded formula =
+  let helper f =
+	match f with
+	| MU (s, f1)
+	| NU (s, f1) ->
+	   (match f1 with
+		| VAR s ->
+		   raise (CoAlgException ("formula not guarded"))
+		| _ -> ())
+	| _ -> ()
+  in
+  iter helper formula;
   true
+
 (** Appends a string representation of a formula to a string buffer.
     Parentheses are ommited where possible
     where the preference rules are defined as usual.