Rspec test coverage on initializer private method on proxy class

I have the following proxy class that I am trying to hit 100% test coverage on. No matter how I construct this Rspec test simplecov reports the klass_constructor method as not covered.

How would I alter these specs to make sure that is covered. I know initializers do not return values i.e. false or true in the rescues but weird events happen with these Rspec stubs.

Proxy Class Code

class IntegrationProvider
  include IntegrationError
  include IntegrationSettings

  def initialize(provider)
    @provider = provider
    return false unless valid_provider?(@provider)
    return false unless valid_klass_constructor?
    klass_constructor
  end

  def proxy_to
    @provider_klass
  end

  private

  def klass_constructor
    @provider_klass = "#{@provider.capitalize}::#{@provider.capitalize}Provider".safe_constantize
  end
end

Included Settings Module

module IntegrationSettings
  def valid_provider?(provider)
    supported_providers.include?(provider)
  end

  def valid_klass_constructor?
    klass_constructor
  end

  private

  def supported_providers
    Rails.cache.fetch('supported_providers', expires_in: 10.days) do
      Provider.where(active: true).pluck(:slug)
    end
  end
end

Spec

RSpec.describe IntegrationProvider, type: :integration do
  let!(:provider) { FactoryGirl.create(:provider, :active) }
  let!(:klass) { Provider }

  describe '#initialize' do
    it 'initializes' do
      stub_const("#{provider.slug.capitalize}::#{provider.slug.capitalize}Provider", klass)
      expect(IntegrationProvider.new(provider.slug)).to be_truthy
    end
  end

  describe '#proxy_to' do
    subject { described_class.new(provider.slug) }

    it 'is a proxy' do
      subject.instance_variable_set(:@provider_klass, klass)
      expect(subject.proxy_to).to eq(klass)
    end

    it 'inherits active record' do
      subject.instance_variable_set(:@provider_klass, klass)
      expect(subject.proxy_to.ancestors.include?(ActiveRecord::Base)).to be_truthy
    end
  end

  describe '#klass_constructor' do
    subject { described_class.new(provider.slug) }

    it 'true' do
      stub_const("#{provider.slug.capitalize}::#{provider.slug.capitalize}Provider", klass)
      expect(subject.send(:klass_constructor)).to be_truthy
    end

    it 'assignment' do
      stub_const("#{provider.slug.capitalize}::#{provider.slug.capitalize}Provider", klass)
      subject.send(:klass_constructor)
      expect(subject.instance_variable_get(:@provider_klass)).to eq(klass)
    end
  end
end

This does not solve the issue, but

"#{@provider.capitalize}::Events::#{@provider.capitalize}Queue".safe_constantize

is cleaner.