sqlx question

I've been attemption to write a very large convoluted query to extract xml out of the database. I've narrowed the problem to the following (abbreviated) piece that i can't seem to figure out why it doesn't work. This is 10r2 on linux. Any help would be greatly appreciated.

  SELECT XMLELEMENT("UnitPData",

		         XMLAGG(XMLFOREST(UNIT_PROGRAM.UP_ID AS "UnitPIDKey"),
		     	               (SELECT XMLELEMENT("UnitPEData",               
       XMLAGG(XMLFOREST(UNIT_PROGRAM_EXEMPTION.UPE_ID AS


"UnitPEIDKey")))

                                FROM UNIT_PROGRAM_EXEMPTION
                                WHERE UNIT_PROGRAM.UP_ID =
UNIT_PROGRAM_EXEMPTION.UP_ID),
				             (SELECT XMLELEMENT("UnitPRFreqData",

                                 FROM UNIT_PROGRAM_REPORTING_FREQ
                                 WHERE UNIT_PROGRAM.UP_ID =
UNIT_PROGRAM_REPORTING_FREQ.UP_ID)
                            )
                    )
      FROM UNIT_PROGRAM;

The error returned is:

ORA-06553: PLS-306: wrong number or types of arguments in call to 'SYS_IXMLAGG'

It seems like a similer construct in an enclosing piece of the sqlx works fine.

chris

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