switch assertion to an explicit ValueError
diff --git a/Lib/lib2to3/fixer_util.py b/Lib/lib2to3/fixer_util.py
index 30da893..350a62a 100644
--- a/Lib/lib2to3/fixer_util.py
+++ b/Lib/lib2to3/fixer_util.py
@@ -274,9 +274,9 @@
"""Find the top level namespace."""
# Scamper up to the top level namespace
while node.type != syms.file_input:
- assert node.parent, "Tree is insane! root found before "\
- "file_input node was found."
node = node.parent
+ if not node:
+ raise ValueError("root found before file_input node was found.")
return node
def does_tree_import(package, name, node):