Rspec and delegate

rspec_and_delegate.rb

require "delegate"
class A < SimpleDelegator
  def ex(foo)
  end
end

rspec_and_delegate_spec.rb

require_relative "rspec_and_delegate"

describe "stub" do
  it "should not raise any error" do
    a = A.new([])
    a.stub(:ex) # it did not actually stub ex method on A
    expect { a.ex }.to_not raise_error # this will raise wrong number of arguments
  end
end

solved this issue by explicitly defining method on an instance by instance_eval.
any ideas how to solve this with rspec?