FNDERROR_MESSGAGENMESSAGE is getting prefixed with Custom error messages when using PO account gener

Hi,
We are displaying/generating custom error message when Account generator fails to derive the code combination based on some business rules.

When we test it using core form , we see the custom error message as expected
But when we use the Buyers work center form (Navigation Buyer work center under purchasing responsibility--> Orders) and try to enter a PO , we get the error message but prefixed with FNDERROR_MESSGAGENMESSAGE.

Screen shots have been enclosed showing the error message displayed using core form and buyer work center form.

Please suggest on how to resolve so that when used from buyers work center , we get just the message and not the message prefixed with FNDERROR_MESSGAGENMESSAGE.