XMLQuery select gives "PLS-00801: internal error"

I take it this means Oracle does not do implicit casting between xmltype and varchar2? I don't see why this is not possible... an implicit cast could just copy the xmltype stuctured data into a varchar2 (or clob).

The version is Oracle 10gR2 Standard running on RHAS4.

"Error(71,2): PLS-00801: internal error [*** ASSERT at file pdw4.c, line 793; Cannot coerce between type 43 and type 30;"

procedure xmltest as

xmldata xmltype := xmltype('<packet><data>some data</data></packet>'); xmlresult varchar2(4000);

begin

select xmlquery('for $a in $doc/packet/data return <data>{$a}</data>' passing xmldata as "doc" returning content) into xmlresult from dual;

end;