XML parsing error b/c of attribute

I'm trying to parse xml that contains attributes which seems to be blowing up my code (sql 2k5 error:'XML parsing: ...undeclared prefix'), what am I doing wrong?
If I remove the bold ' xsi:nil="true" ' the code is great, but the real xml file has attributes all over the place so I'm sure there is a way I'm just not familiar enough with XML to know.

DECLARE @x xml;
Set @x= '<?xml version="1.0"?> <main> <whyMe xsi:nil="true" /> <test> <row> <a>1</a> <b>2</b> </row> </test> </main> '

DECLARE @iDoc int;
EXEC sp_xml_preparedocument @iDoc OUTPUT, @x;

SELECT * FROM OPENXML(@iDoc, '//row', 2) WITH (a int);

EXEC sp_xml_removedocument @iDoc;
TIA!!!
Stefano