Angular Error: Illegal state: Could not load the summary for directive SomeComponent

Apparently I had encountered this issue before, but it came up again and I had to research it again (I only know I came across it before because the link to the answer on SO was purple).

When your tests fail with this message, make sure you're including the component under test (SomeComponent) in the declarations of the TestBed. It's super simple, but apparently it's bitten me at least twice.