I'm trying to use the stock store module to sell tickets to an event.  I have everything configured correctly to the point with Authorize.net that transactions are going through and the store administrator receives confirmation of the order with all the billing and shipping details (in test mode).  However, the end user would never know that a transaction was completed because the following error is displayed after submitting the order:

A critical error has occurred.
Value cannot be null. Parameter name: format

I would assume that there should be a "thank you for your order" page set up somewhere to which the module would automatically refer a user.  But how/where is that page conctructed and configured?