Url for msg.post can be nil.
It is very easy to make misprint and use nil instead of needed value.
That things is easy to make, but hard to find. If there are some warnings about nil in msg post it can help find that problem.
Example from real life
--was
msg.post(self.url, "set_time_step", {factor = 0, mode = 0})]
--should be
msg.post(self._url, "set_time_step", {factor = 0, mode = 0})