Adding a custom patrol to AE mission

I tried to add a patrol of custom mobs to an AE mission today and couldn't because I could not select the custom group I wanted to.

I have two custom groups. Everything else in the mission is from my Diggers group. When I tried to select that group to indicate what should be in the patrol, it wasn't an option. My other group and all custom critters were options or standard groups. Couldn't leave the group as 'none' as that kicked out an error.

Any ideas as to why?