I just fixed this problem by installing the pymodbus 1.5.0.

It was reported as a bug few time ago upon 1.4 (link) and it was just fixed (link)

 

If the below command  gives you the 1.4.0 version of pymodbus

pip show pymodbus 

You have to install the 1.5

sudo pip install pymodbus==1.5.0

 

BR