Strange exception of DTE.SeletedItems.Item()

[whyithappen]

I am building a add-in project to extend the VS
environment.

Code excerpt:

EnvDTE.DTE hostApplication;
....
SelectedItems selItems = hostApplication.SelectedItems;
SelectedItem selItemObj = null;
int count = selItems.Count;
if(( count > 0)&&(!selItems.MultiSelect))
{
try
{
selItemObj = selItems.Item(1);
}
catch(Exception e)
{
string s = e.ToString();
}
...
}

Result:

For most of the projects opened or selected,the code runs
well;for quite few projects,exception catched while
calling selItems.Item(1),and s turns out to be:
"System.Runtime.InteropServices.COMException (0x80020006):
Unknown name.\r\n at ...";and once this exception
occurs,it occurs on all projects opened.

Could anybody tell me how to solve this problem?Thanks

Whyithappen