ROS2, TypeError when publishing custom message to Topic

edit

Hi everyone,

I have defined a custom message:

uint8[] data

The custom message is imported in my Node class with no problems:

from my_shared.msg import MyMessage

In the same Node, I create the publisher with:

self.my_publisher = self.create_publisher(MyMessage, 'topic_in', 200)

and I publish the message with:

self.my_publisher.publish(my_msg)

my_msg is built in the following way:

payload_bitstream = np.fromstring(my_data, np.uint8)
my_msg = payload_bitstream.tolist()

Sadly, I get a TypeError:

File "/opt/ros/eloquent/lib/python3.6/site-packages/rclpy/publisher.py", line 68, in publish
    raise TypeError()
TypeError

Could you help out with this if you know what I am doing wrong pls?

Thanks in advance, G.