diff --git a/src/lib/CoAlgMisc.ml b/src/lib/CoAlgMisc.ml
index 0332f394c5f5155c8ccd883f2a16eef8f2750dcb..8fd1f68135254e104f961b5a58680c8df63ef81e 100644
--- a/src/lib/CoAlgMisc.ml
+++ b/src/lib/CoAlgMisc.ml
@@ -1207,6 +1207,7 @@ let ppFormulae nomTbl tbox (s, f) =
   if card <= 0 then
     raise (C.CoAlgException "Number of sorts must be positive.")
   else ();
+  C.verifyFormula f;
   let nnfAndSimplify f = C.simplify (C.nnf f) in
   let  f1    = nnfAndSimplify f in
   let nf1    = nnfAndSimplify (C.NOT f) in