diff -r 22378817612f -r 24b5e7579fdd src/HOL/Library/Multiset.thy
--- a/src/HOL/Library/Multiset.thy	Tue Oct 06 11:34:07 2015 +0200
+++ b/src/HOL/Library/Multiset.thy	Tue Oct 06 11:50:23 2015 +0200
@@ -2048,7 +2048,7 @@
               | NONE => [Const (maybe_name, elem_T --> elem_T) $ t])
           in
             (case maps elems_for (all_values elem_T) @
-                 (if maybe_opt then [Const (Nitpick_Model.unrep (), elem_T)] else []) of
+                 (if maybe_opt then [Const (Nitpick_Model.unrep_mixfix (), elem_T)] else []) of
               [] => Const (@{const_name zero_class.zero}, T)
             | ts =>
                 foldl1 (fn (t1, t2) =>