Verification Of Payee API Specifications lists some of the error cases identified and provides the corresponding error codes. However, there is not a message code in two cases (Certificate items and Internal Server Error).

We believe that this is a mandatory field consequently it should be added specific message code. Could you please clarify what code should be used in these cases?
In these cases, the response code is sufficient and self-explanatory, there is no need for a detailed error code.  

Only in certain cases can additional error information be added. 

The two options namely, “no message code” and “message with an empty message code” are acceptable, meaning means that developers should be prepared to receive both types of error message.