xpath starts-with函数出错！大家帮忙,该怎么解决

xpath starts-with函数出错！大家帮忙
我的xpath是这样写的：

xpath   :=   '/Documents/Document[starts-with(@Index,   ' 'D ' ')] ';

然后调用：SelectSingleNode(xpath)

运行时出错：Unknow   Method
请帮忙啊～～

给出代码片断供参考：
-----------------------------
//   Delphi
var
    xpath:   string;
    node:   IXMLDOMNode;
begin
    xpath   :=   '/Documents/Document[starts-with(@Index,   ' 'D ' ')] ';
    node   :=   doc.SelectSingleNode(xpath);
end;

----------------------------
//   c#
string   xpath   =   "/Documents/Document[starts-with(@Index,   ' 'D ' ')] ";
XmlNode   node   =   doc.SelectSingleNode(xpath);



[解决办法]
string xpath = "/Documents/Document[starts-with(@Index, 'D ')] ";
O.o