User Search Code Throws on Vista

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I am trying to create a user on a Vista Ultimate system, only if it
does not exist. This code throws:

"
The user name could not be found. (Exception from HRESULT: 0x800708AD)
"

I know the user name could not be found - that is why I am checking!
What is the proper syntax to let this check properly work.

Thanks.

string local = "WinNT://" + Environment.MachineName + ",computer";
DirectoryEntry root = new DirectoryEntry( local );
DirectoryEntry entry = null;
if (root.Children.Find("TestAccount","User") == null) // <-throws
{
// ...make new user...
}

..NET Framework 3.5 Console app.

.