c# - Microsoft.Xml.XQuery math expression error -

i'm newer in microsoft.xml.xquery, when i'm trying run xquery math expression sum have error : cannot apply add non integer data.

xml:

<book category="cooking">   <title lang="en">everyday italian</title>   <author>giada de laurentiis</author>   <year>2005</year>   <price>30.00</price> </book> 

xquery:

for $item in document("xqt")/bookstore/book[price < '30'] return $item/year/text()  +  $item/year/text()  

c#:

  xmldocument doc1 = new xmldocument();         doc1.loadxml(rtexml.text);         xquerynavigatorcollection navcol = new xquerynavigatorcollection();         navcol.addnavigator(doc1.createnavigator(), "xqt");          xqueryexpression expr = new xqueryexpression(rtexquery.text.trim().replace("\\n", " "));          xquerynavigator nav = expr.execute(navcol);          rteoutput.text = nav.toxml(); 

any ideas?

for $item in document("xqt")/bookstore/book[price < '30'] return sum($item/year/text(), $item/price/text())