SQL & PL/SQL

Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
with the partitioning, real application clusters, olap, data mining
and Real Application Testing options

Working with relational tables containing clob type columns (no XMLDB available) I'm not able to set up a simple validation example as below (supposed to return 1 confirming a successful validation).

declare 
  xmldoc xmltype;
  sb_xml xmltype;
  valid_ number;
  xmlxsd varchar2(4000) := q'~<?xml version="1.0" ?>
                              <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
                              <xs:element name="greeting" type="xs:string" />
                              </xs:schema>~';
  xmlxml varchar2(32767) := q'~<?xml version="1.0" ?><greeting>"HELLO !"</greeting>~';
begin 
  xmldoc := xmltype(xmlxml);
  sb_xml := xmldoc.createschemabasedxml();
  valid_ := sb_xml.isschemavalid(xmlxsd);
  dbms_output.put_line('IsSchemaValid (0=false,1=true): ' || to_char(valid_));
end;

Thanks in advance for any kind of help provided

Etbin