diff -r 83b3c110f22d -r a2dd9200854d src/HOL/Tools/Nitpick/nitpick_model.ML
--- a/src/HOL/Tools/Nitpick/nitpick_model.ML	Sat Mar 22 18:16:54 2014 +0100
+++ b/src/HOL/Tools/Nitpick/nitpick_model.ML	Sat Mar 22 18:19:57 2014 +0100
@@ -1053,7 +1053,7 @@
                     (Object_Logic.atomize_term thy prop)
         val prop = HOLogic.mk_Trueprop (HOLogic.mk_imp (assm, concl))
                    |> map_types (map_type_tfree
-                                     (fn (s, []) => TFree (s, HOLogic.typeS)
+                                     (fn (s, []) => TFree (s, @{sort type})
                                        | x => TFree x))
         val _ =
           if debug then