message: A tiny change for better err msg
diff: @@ -875,7 +875,7 @@ def func_graph_from_py_func(func, arg_names, arg_types, name=None,
  if not isinstance(outputs, (list, tuple)):
  outputs = (outputs,)
  if any([_ is None for _ in outputs]):
  - raise ValueError("Function can not return None.")
  + raise ValueError("Function %s can not return None." % name)
  # Ensures each output is a Tensor in the function graph.
  outputs = [ops.convert_to_tensor(t) for t in outputs]
  outputs = [func_graph.capture(t) if t.graph is not func_graph else t
  