System error -2146697210

我在用C++写一个用MSXML解析XML文件的程序，文件load没有问题，但是在用selectSingleNode选取节点时总是得不到节点，返回错误System error -2146697210，请各位大侠帮忙，不胜感激。。。

no code, no truth！

#import <msxml4.dll> 
#include <string>
#include <list>
#include <objbase.h>
using namespace MSXML2;

CoInitialize(NULL); //初始化COM库

HRESULT hr;
MSXML2::IXMLDOMDocument2Ptr ptrFedDocument=NULL; //指向整个FED文件的指针
MSXML2::IXMLDOMElementPtr ptrElement=NULL; 
MSXML2::IXMLDOMNodePtr ptrRootNode=NULL; //整个文件的根节点 

hr=ptrFedDocument.CreateInstance(__uuidof(DOMDocument40));//创建MSXML文档实例
if(FAILED(hr))
{
CoUninitialize();
throw FSLLRC_CreateMSXMLInstanceException();
}

ptrFedDocument->async = VARIANT_FALSE;

hr=ptrFedDocument->load(fedFileName.c_str()); //加载FED文件

if(FAILED(hr))
{
CoUninitialize();
throw FSLLRC_LoadFEDException();
}

ptrElement=ptrFedDocument->GetdocumentElement();
ptrRootNode=ptrFedDocument->selectSingleNode(_bstr_t("FSLRTIFED"));
if(NULL==ptrRootNode)
{
BSTR reason;
ptrFedDocument->parseError->get_reason(&reason);
system("pause");
}
if(NULL==ptrRootNode)
{
if(ptrFedDocument!=NULL)
{
ptrFedDocument.Release();
}
if(ptrElement!=NULL)
{
ptrElement.Release();
}
CoUninitialize();
throw FSLLRC_FEDDetialException("FED文件根节点为空");
}
 
if(ptrFedDocument!=NULL)
{
ptrFedDocument.Release();
}
if(ptrElement!=NULL)
{
ptrElement.Release();
}
if(ptrRootNode!=NULL)
{
ptrRootNode.Release();
}
CoUninitialize();

这是程序中解析部分的代码，
<?xml version="1.0" encoding="UTF-8"?>
<!--FED file template-->
<FSLRTIFED>
  <Federation/>
  <Fedversion/>
  <spaces/>
  <ObjectRoot name="FSLRTIObjectRoot">
  <Object name="Plan">
  <attribute name="PositionX" TransportationType="Reliable" TransportationOrder="TimeStamp"/>
  <attribute name="PositionY" TransportationType="Reliable" TransportationOrder="TimeStamp"/>
  <attribute name="PositionZ" TransportationType="Reliable" TransportationOrder="TimeStamp"/>
  <Object name="K-8">
  <attribute name="K-8-Color" TransportationType="Reliable" TransportationOrder="TimeStamp"/>
  <Object name="K-8-2">
  <attribute name="K-8-2-Speed" TransportationType="BestEffort" TransportationOrder="TimeStamp"/>
  <attribute name="K-8-2-Acceleration" TransportationType="BestEffort" TransportationOrder="TimeStamp"/>
  </Object>
  </Object>
  </Object>
  </ObjectRoot>
  <InteractionRoot name="Bing" TransportationType="Reliable" TransportationOrder="ReceiveOrder">
  <parameter name="BoulNum"/>
  <parameter name="DX"/>
  <parameter name="DY"/>
  </InteractionRoot>
</FSLRTIFED>
这是XML文件