switch assertion to an explicit ValueError
diff --git a/Lib/lib2to3/refactor.py b/Lib/lib2to3/refactor.py
index badcac2..a4c168d 100644
--- a/Lib/lib2to3/refactor.py
+++ b/Lib/lib2to3/refactor.py
@@ -445,7 +445,7 @@
 
                         try:
                             find_root(node)
-                        except AssertionError:
+                        except ValueError:
                             # this node has been cut off from a
                             # previous transformation ; skip
                             continue