cannot send or receive an unsupported LOB

Hi,
I new in oracle
I have querry for conver from oracle to xml file
select dbms_xmlgen.getxml('SELECT ALL MAITRE_APPL.MRC_TYPE_TRANSACTION_DETAIL.NUMERO,
MAITRE_APPL.MRC_TYPE_TRANSACTION_DETAIL.DESCRIPTIO N
FROM MAITRE_APPL.MRC_TYPE_TRANSACTION_DETAIL') xml
from dual;
but this is querry give me error:
ERROR at line 3:
ORA-24813: cannot send or receive an unsupported LOB
Thanks