diff --git a/mercurial/dirstatemap.py b/mercurial/dirstatemap.py
--- a/mercurial/dirstatemap.py
+++ b/mercurial/dirstatemap.py
@@ -733,7 +733,3 @@
 
         def _drop_entry(self, f):
             self._map.drop_item_and_copy_source(f)
-
-        def __setitem__(self, key, value):
-            assert isinstance(value, DirstateItem)
-            self._map.set_dirstate_item(key, value)