My exception handler is not catching this message. The error occurs
because the unique constraint is being violated. Instead of getting an
-803 SQLCODE, I'm getting 'no current record for fetch operation'

ISC ERROR CODE:335544348
ISC ERROR MESSAGE:
no current record for fetch operation
violation of PRIMARY or UNIQUE KEY constraint "U_RPLCLMPOBILLITM_PK"
on table "RPL_CLM_POBILL_ITM"

BEGIN
INSERT INTO RPL_CLM_POBILL_ITM (
POBILL_ITM,
CLAIM_ID,
QTY_CLAIM,
QTY_SHIP,
CREATED)
VALUES (
:POBILL_ITM,
:CLAIM_ID,
:QTY_CLAIM,
:QTY_SHIP,
:POSTED);
WHEN SQLCODE -803 DO
UPDATE RPL_CLM_POBILL_ITM SET
QTY_CLAIM = QTY_CLAIM + :QTY_CLAIM,
QTY_SHIP = QTY_SHIP + :QTY_SHIP
WHERE
POBILL_ITM = :POBILL_ITM AND
CLAIM_ID = :CLAIM_ID AND
GRANTEE_INVC IS NULL;
END

The server version is LI-V6.3.1.4481 Firebird 1.5