[SOLVED] No plugin to handle input format: doc

I am trying to convert a doc to an epub using calibre via the command line but I get this error.

Traceback (most recent call last):
  File "site.py", line 61, in main
  File "site-packages/calibre/ebooks/conversion/cli.py", line 341, in main
  File "site-packages/calibre/ebooks/conversion/cli.py", line 300, in create_option_parser
  File "site-packages/calibre/ebooks/conversion/plumber.py", line 734, in __init__
ValueError: No plugin to handle input format: doc

I haven't been able to find any answers or documentation about this.

I am using shell_exec to run this command via PHP.

I am using the latest release of calibre which is 1.45.0.

No plugin to handle input format: doc - Calibre