entityreference field attached to payment, errors

i want to make a reference to node with payment form from payment. But, after attach a entityreference field to payment form, select any option and save:

Fatal error: Cannot unset string offsets in /***/modules/field/field.module on line 499

this is becouse, this field form_state['value] is saved as ['values]['field_ref'] = $value, and it must be saved as ['values]['field_ref'][0] = $value;

thank you