display_list must return str, not unicode

tiramisu/error.py

@@ -26,7 +26,10 @@ def display_list(lst, separator='and'):
     if len(lst) == 0:
         return ''
     elif len(lst) == 1:
-        return lst[0]
+        ret = lst[0]
+        if isinstance(ret, unicode):
+            ret = ret.encode('utf8')
+        return ret
     else:
         lst_ = []
         for l in lst[:-1]: