webrequest.create（URL）中的NUll引用异常

Question

我在行webrequest.create(v_strURL)

中得到一个空引用异常

Dim objHttpWebRequest As HttpWebRequest
        Dim objHttpWebResponse As HttpWebResponse
        Dim objRequestStream As Stream = Nothing
        Dim objResponseStream As Stream = Nothing

        Dim objXMLReader As XmlTextReader

        objHttpWebRequest = CType((WebRequest.Create(v_strURL)), HttpWebRequest) ''This line throws exception.

Url是正确的，并且确实给出了正确的答案。