Musepack on LMS on a Raspberry Pi

In my LMS settings, on the Advanced > File Types page, there's an entry for Musepack, but all of the dropdown menus are set to "Disabled" and greyed-out (and therefore unchangeable)



How do I enable Musepack?

I'm running LMS 7.9.2 on Raspbian 9.8 Stretch.

TIA.

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