TypeError during assert_no_difference test

Question

The issue
I am writing unit tests for a RoR API, and I would like to test some entries from the response's body (something I have already done for another controller).
Basically, for an index method, I wanna check that every sub-element included in data share the same id as a specific one. Here is the complete test for the index method :

  test "should access Membership index - specifix team" do
    get api_v1_team_memberships_url(@team),
    headers: { Authorization: JsonWebToken.encode(user_id: @user.id) }, 
    as: :json
    assert_response :success
   
    json_response = JSON.parse(self.response.body)
    assert_no_difference @membership.member_id, json_response['data']['attributes']['member_id']
  end

But whatever I try, I get the very same error stack saying I have an issue with data type :

Error:                                                                                                    
Api::V1::MembershipsControllerTest#test_should_access_Membership_index_-_specifix_team:                   
TypeError: no implicit conversion of String into Integer                                                  
    test/controllers/api/v1/memberships_controller_test.rb:20:in `[]'                                     
    test/controllers/api/v1/memberships_controller_test.rb:20:in `block in <class:MembershipsControllerTes
t>'

I really don't get why, since, as I said above, that's a check I've already done for some other controllers.

assert_equal @task.activity_id, json_response['data']['attributes']['activity_id']

What I have tried so far

• convert both element from the assert_no_difference function to string (to_s) or to integer (to_i)
• use assert_equal instead of assert_no_difference