connecting a python block with a cpp block makes gnuradio stop

Question

I have two gnuradio OOT blocks. When connected to each other, my flowgraph stops almost immediately without displaying any error, as follow.

Generating: '/root/top_block.py'

Executing: /usr/bin/python -u /root/top_block.py

*Two or three logs that match starting procedure of both blocks*

>>> Done

When the two blocks are disconnected, or used separately, everything works fine.

The first one is written in python and output out_sig=[np.complex64]

The second one is written in C++ and input io_signature::make(1, 1, sizeof(gr_complex))

I have experienced a similar problem while connecting two OOT blocks with pmt messages, and the reason was a different type of pmt message as output/input. This is why I'm looking at the type of data being exchanged. However it says here that gr_complex and numpy.complex64 are equivalent so maybe I'm not looking in the right direction.