diff --git a/sloth/items/factory.py b/sloth/items/factory.py
index b511600..6f3002d 100644
--- a/sloth/items/factory.py
+++ b/sloth/items/factory.py
@@ -52,10 +52,10 @@ class Factory:
             Type for which the mapping should be removed.  If None, all
             mappings will be removed.
         """
-        _type = str(_type)
         if _type is None:
             self._items = {}
         else:
+            _type = str(_type)
             if _type in self._items:
                 del self._items[_type]