How to use shoulda-matcher to test uniqueness of one foreign key scoped to the other?

I know that this question exists, but the answers didn't help me.

Working with the Participant Model, which

belongs_to :game
belongs_to :team

The shoulda-matcher code that I need to work is

it {should validate_uniqueness_of(:team).scoped_to(:game)

I have tried to follow the answers at the above question and the solutions on the git hub itself for this known issue, and gotten no luck.

If I just leave it 'clean' I get this error:

 Failure/Error: it {should validate_uniqueness_of(:team).scoped_to(:game)}

 ActiveRecord::AssociationTypeMismatch:
   Team(#70267561823460) expected, got String(#70267552833620)

To complicate things just a little, Team belongs_to :division

I'm using rspec and rails 5. Is there an answer within shoulda-matchers is the question?