Can I use the word object from within a webservice?

I'm trying to do a proof of concept to see if I can do some very basic word automation from within a webservice.  The problem I have is that after I have created an instance of the Word Appliation I am unable to open any files.  The Document that I am trying to get is always null.  Any ideas whats wrong? Is what I'm trying to do even possible?

In my code below you'll see that all I'm trying to do is create an instance of word (which succeeds), and then open the document (which fails and gives me a null reference). (The doc.Activate is only to cause an the null reference error)

private void PrintDocument(string safeFileName)
{
	Microsoft.Office.Interop.Word.Application app = new Microsoft.Office.Interop.Word.Application();
	// Open Doc
	object filename = safeFileName;
	Microsoft.Office.Interop.Word.Document doc = app.Documents.OpenNoRepairDialog(ref filename, ref _missing, ref _missing, ref _missing, ref _missing, ref _missing, ref _missing,
																											ref _missing, ref _missing, ref _missing, ref _missing, ref _missing, ref _missing, ref _missing,
																											ref _missing, ref _missing);
	doc.Activate(); // fails here because doc == null
	app.Quit(ref _missing, ref _missing, ref _missing);
}

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