Rails 5 upgrade - Failing controller tests

I'm reasonably new to Rails and have just started working on a large project that needs updating from Rails 4.2 to Rails 5. I've never used RSpec much before, and I'm running into some failing tests while attempting the upgrade.

The code for the tests is:

describe "#index " do
  it 'should return all plots associated to a purchaser' do
    sign_in @user

    get purchaser_plots_path(format: :json), params: {}, flash: {'Accept' => Mime::JSON}

    _(response.status).must_equal 200
    json = JSON.parse(response.body)
    _(json.count).must_equal 2
    _(json.first['id']).must_equal @plot_2.id
    _(json.second['id']).must_equal @plot_1.id
  end

  it 'should paginate' do
    sign_in @user

    get purchaser_plots_path(format: :json), params: {page: 1, per_page: 1}, flash: {'Accept' => Mime::JSON}
    _(response.status).must_equal 200
    json = JSON.parse(response.body)
    _(json.count).must_equal 1
    _(json.first['id']).must_equal @plot_2.id

    get purchaser_plots_path(format: :json), params: {page: 2, per_page: 1}, flash: {'Accept' => Mime::JSON}
    _(response.status).must_equal 200
    json = JSON.parse(response.body)
    _(json.count).must_equal 1
    _(json.first['id']).must_equal @plot_1.id
  end
end

This is giving me the following error:

Error:
PurchaserPlotControllerTest::#index #test_0001_should return all plots associated to a purchaser:
ArgumentError: unknown keyword: flash
    test/controllers/purchaser/plots_controller_test.rb:21:in `block (2 levels) in <class:PurchaserPlotControllerTest>'

All of the help I've found online seems to state that 'flash' is an allowed keyword argument, so I'm not sure what I'm doing wrong!

Any help would be greatly appreciated.