Index: Lib/lib2to3/pytree.py
===================================================================
--- Lib/lib2to3/pytree.py	(Revision 74653)
+++ Lib/lib2to3/pytree.py	(Arbeitskopie)
@@ -130,6 +130,8 @@
 
     def replace(self, new):
         """Replace this node with a new one in the parent."""
+        if self is new:
+            return
         assert self.parent is not None, str(self)
         assert new is not None
         if not isinstance(new, list):
@@ -147,9 +149,12 @@
         assert found, (self.children, self, new)
         self.parent.changed()
         self.parent.children = l_children
+        # one of the new nodes may be self;
+        # make sure it doesn't lose its parent.
+        parent = self.parent
+        self.parent = None
         for x in new:
-            x.parent = self.parent
-        self.parent = None
+            x.parent = parent
 
     def get_lineno(self):
         """Return the line number which generated the invocant node."""