Custom component of existing integration not loaded

I have several custom components/integrations in my /config/custom_components folder of my RPi (running Home Assistant OS).
One of these is a replacement for the existing modbus integration with some additional files
(e.g. cover.py, light.py).

But it looks like this integration isn’t loaded correctly:

Platform error light.modbus - No module named 'homeassistant.components.modbus.light'
Platform error cover.modbus - No module named 'homeassistant.components.modbus.cover'

Any idea what’s the problem?

Regards,
Daniel.