A function that is supposed to return something should return something.

Cython/Compiler/Code.py

@@ -437,7 +437,7 @@ class UtilityCodeBase(object):
         return "<%s(%s)>" % (type(self).__name__, self.name)
 
     def get_tree(self, **kwargs):
-        pass
+        return None
 
     def __deepcopy__(self, memodict=None):
         # No need to deep-copy utility code since it's essentially immutable.