XML Creation - Receieving ORA-19206: Invalid value for query or REF CURSOR parameter error

 I am new to XML creation, I am running this simple query in TOAD. It all works fine until I add a cursor to it then receive the errors mentioned above. What do these mean and how do I fix it? Thank you very much in advance.

Here is the query

SELECT DBMS_XMLGEN.GETXML ('

select poh.segment1 blanket,poh.comments,poh.start_date,poh.end_date,

CURSOR(select pol.line_num from po_lines_all pol where pol.po_header_id = poh.header_id) LINE_INFO

from po_headers_all poh')XML

DBMS_XMLGEN.GETXML (' select poh.segment1 blanket,poh.comments,poh.start_date,poh.end_date, CURSOR(select pol.line_num from po_lines_all pol where pol.po_header_id = poh.header_id) LINE_INFO from po_headers_all poh'