Change payment option on review page

When i try to change the payment option on review page an ajax request will be started and i get the following error. There are only two payment methods, a "cash on delivery" and a "cash" payment created by payment module.

error in popup:

Notice: Undefined index: payment_method in commerce_payment_pane_checkout_form() (line 88 of commerce/modules/payment/includes/commerce_payment.checkout_pane.inc).

errors in firebug console:

array(
['type'] =>
'rules'
['message'] =>
'Unable to get a data value. Error: Missing data values.'
)

array(
['type'] =>
'rules'
['message'] =>
'Unable to evaluate condition payment_commerce_payment_method_validate.'
)

I am using commerce 1.8, rules 7.x-2.6+1-dev, payment 7.x-1.9+3-dev. Thx