Instance variable attributes() method in tests fixtures

as stated in Rails Book example , I have a fixtures file orders.yml

id: 1
name: Fred
email_address: fred@flinstone;com

in my controller test file, I tried :
fixtures :orders
def test_save_valid_order
post :save_order, :order => @valid_order_fo_fred.attributes

but I get an error

NoMethodError: You have a nil object when you didn’t expect it!
You might have expected an instance of ActiveRecord::Base.
The error occurred while evaluating nil.attributes