When you are homebrewing a monster, if you have a Monster Type of ‘unknown’ it will throw an error when you try to view that monster in an encounter.

This would be after an encounter has been saved and you are ready to run the encounter and you click on the monster to show its statistics. Other than the D&D Beyond header, there is only a “Report Feedback” button, which doesn’t work.

Also, the icon for an ‘unknown' monster doesn’t display. It shows an empty graphical placeholder instead.

There is no official “Unknown” monster type, that’s probably why. I wonder what that’s doing in there, it must be new.

Here is how you can reproduce the error:

1. Create a new monster using an existing monster as a template. For this example I used the first one on the list “The Demogorgon”
2. Change its name (“A Bad Monster”) and change the type (“unknown”). Save it.
3. Create a new encounter (“A Bad Encounter”). Add “A Bad Monster” to it. Save it.
4. When you click on the monster in the encounter (either before running it or during running it), it throws a “Report feedback” error

Yes, I know this is not an official monster type. However, I'm creating my own monster template and having an unspecified monster type is useful to me.

This thread talks about the "unknown" monster type: https://www.dndbeyond.com/forums/d-d-beyond-general/bugs-support/38522-unknown-monster-type-in-monster-creator