How to use Rails 4 strong parameters with has_many :through association?

I'm having trouble getting a has_many :through association working with Rails 4's strong parameters. I have a model called Checkout and I need to select a person from the Employee model in the new checkout form. Checkouts and Employees are associated through an Employment model. I'm getting this error when I try to create a new checkout: NoMethodError in CheckoutsController#create undefined method