problem with build_source

gistfile1.rb

context "#build_source" do
  let(:payment_method) { Factory(:bogus_payment_method) }
    
  it "should build the payment's source" do
    params = { :amount => 100, :payment_method_id => payment_method.id,
      :source_attributes => {:year=>"2012", :month =>"1", :number => '1234567890123',:verification_value => '123'}}
     
    payment = Spree::Payment.new(params)      
    payment.should be_valid
    payment.source.should_not be_nil
  end
    
  context "with the params hash ordered differently" do
    it "should build the payment's source" do
      params = {
        :source_attributes => {:year=>"2012", :month =>"1", :number => '1234567890123',:verification_value => '123'},
        :amount => 100, :payment_method_id => payment_method.id
      }  
     
      payment = Spree::Payment.new(params)      
      payment.should be_valid
      payment.source.should_not be_nil
    end     
  end
end

ruby 1.8.7p352 - both tests pass
ree - sometimes fails, sometimes passes
ruby 1.8.7p357 - sometimes fails, sometimes passes
ruby 1.9.2p290 - 1st test passes, 2nd test fails
ruby 1.9.3p0 - 1st test passes, 2nd test fails