Payment Processor Error message: 10525: Invalid Data This transaction cannot be processed. The amount to be charged is zero
Update your site to CiviCRM 4.6.7

This was a bug, reported at https://issues.civicrm.org/jira/browse/CRM-14536

We had this problem on our site, and it was fixed as of 4.6.7.  Update your site to the latest version, and you should be all set!