[Rails] Activemerchant rails 3.1 wrong number of arguments

Hi,

Ive followed the railscasts to integrate paypal express and
activemerchant on rails 2.5 without any issues
http://railscasts.com/episodes/146-paypal-express-checkout.

When using the same code in a rails 3.1 app i get the following error
below, this happens when i try to set the purchase up:

EXPRESS_GATEWAY.setup_purchase(100, {:return_url =>
"www.bbc.co.uk", :cancel_return_url => "www.bbc.co.uk"})

ArgumentError: wrong number of arguments (1 for 0)
from /Users/jxxxxxxxxxr/.rvm/gems/
ree-1.8.7-2010.02@mystaffdeals/gems/
builder-3.0.0/lib/builder/xmlbase.rb:135:in `to_xs'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
builder-3.0.0/lib/builder/xmlbase.rb:135:in `_escape'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
builder-3.0.0/lib/builder/xmlbase.rb:140:in `_escape_quote'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
builder-3.0.0/lib/builder/xmlmarkup.rb:320:in `_attr_value'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
builder-3.0.0/lib/builder/xmlmarkup.rb:311:in `_insert_attributes'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
builder-3.0.0/lib/builder/xmlmarkup.rb:310:in `each'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
builder-3.0.0/lib/builder/xmlmarkup.rb:310:in `_insert_attributes'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
builder-3.0.0/lib/builder/xmlmarkup.rb:293:in `_start_tag'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
builder-3.0.0/lib/builder/xmlbase.rb:60:in `method_missing'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
builder-3.0.0/lib/builder/xmlbase.rb:34:in `tag!'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
activemerchant-1.18.1/lib/active_merchant/billing/gateways/
paypal_express.rb:97:in `build_setup_request'
from /Users/xxxxxxxx/.rvm/gems/ree-1.8.7-2010.02@mystaffdeals/
gems/
activemerchant-1.18.1/lib/active_merchant/billing/gateways/
paypal_express.rb:25:in `setup_purchase'

Ive also followed this tutorial
http://spin.atomicobject.com/2011/10/24/integrating-paypal-express-with-rails-3-1-part-1/
and get the same error message above when i try the
gateway.setup_purchase method.

Anybody got any ideas, im really stuck now. Ive tried everything i
can think off, different versions of the activemerchant gem, etc etc
etc

JB