"TypeError: data type not understood" when writting block in python

Hi all,

this thread is more or less extension of the thread:
http://gnuradio.4.n7.nabble.com/3-6-4-2-test-fail-tc43452.html

It looks that my 3.6.4git versio doesn’t support message passing logic
within python, so I switched to another pc where I run 3.7.0. I assume
that
because there is no function for registering message ports, but when I
changed to 3.7.0, there was no that error any more.
No the problem is the following:

the block is very simple:

def __init__(self):
    gr.sync_block.__init__(self,
        name="msg_sender",
        in_sig=[numpy.int8],
        out_sig=[None])

    self.message_port_register_out(pmt.pmt_intern('out_port'))

but when i run flowgraph, the following error comes out:
RuntimeError: msg_sender(2): insufficient connected output ports (1
needed,
0 connected)

From where came that output that I havent declared?