Microsoft Word 14.0 Object Library: Application.Documents.Add() return null

Hi,
This issue has troubled me for a few days.
My code:
wordApp = new Word.Application();
Word._Document newDocument = null;
newDocument = wordApp.Documents.Add();
In my local computer, the result is right. But when I deploy the asp.net application in the server, I found that newDocument is null and the wordApp is initialized successfully.
Could you please give me some help?
Thank you very much.

Best regards.
Li JiaWei
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