TESSITURA_ACCESS_DENIED_EXCEPTION Error calling OrderExists

In the Web API Documentation, V9, the overview for the OrderExists function indicates it verifies that an order exists for the specfied session, and returns true or false.

The two errors listed are TESSITURA_INVALID_SESSION_EXCEPTION and TESSITURA_ACCESS_DENIED_EXCEPTION.  I am getting the access denied exception with text that the method called requires the user to be logged in.  The documentation does not say anything about being logged in.  I am passing in a valid session key.

Obviously, it appears that you must be logged in, but will the anonymous login be fine for this?

Thanks.

Paul