No execution plan for MERGE statement. (4272934)

The Explain plan under Toad is not shown when a MERGE statement is used. No error message is returned.  

For example see below:

MERGE INTO SCHEMA.TABLE ABCD
USING (
SELECT
ID_FIELD1, ID_FIELD2, ID_FIELD3
FROM SCHEMA.TABLE C
INNER JOIN SCHEMA.TABLE2 D ON
C.FK_FIELD1 = D. FK_FIELD1
AND C. FK_FIELD2 = D. FK_FIELD2
WHERE
C.DATETIME < (CURRENT TIMESTAMP - 128 DAY)
) CON
ON (
DES. ID_FIELD1 = CON. ID_FIELD1
AND DES. ID_FIELD2= CON. ID_FIELD2
AND DES. ID_FIELD3= CON. ID_FIELD3)
WHEN MATCHED THEN

DELETE ;