Msg 9420, Level 16, State 1, Line 6 XML parsing: line 2, character 33, illegal xml character

Hi,When I run an SQL query, sometimes I got the error,[quote]Msg 9420, Level 16, State 1, Line 6XML parsing: line 2, character 33, illegal xml character[/quote]and sometimes is okay. What would be the potential cause of this error?Thanks,