if max_pool_size is set to None, then if _return_socket is ever called an exception is thrown. The offending code is in pool.py in the _return_socket method.

The line:-

if (len(self.sockets) < self.max_size and sock_info.pool_id == self.pool_id):

throws:-

TypeError: unorderable types: int() < NoneType()

This is easy to work around as just need to set the pool size although the problem is occurring in a third party library I am using because its using the depreciated Connection class which defaults to max_pool_size of None.