error using mitsub_vfd module

I want to use the module for mitsubishi vfd, but get this error:
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Traceback (most recent call last):
File "/usr/bin/mitsub_vfd", line 39, in <module>
import serial
ImportError: No module named serial
./spindle.hal:4: waitpid failed mitsub_vfd coolant
./spindle.hal:4: mitsub_vfd exited without becoming ready
2251
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what serial module do I need to install?

Package python-serial on my Mint system

Chris M