null value in query string

I'm using webservice to integrate asp.net app with PeopleSoft. Sometimes, the value that I get from a querystring (eg in url below,  request id =7) and pass it to PeopleSoft via webservice is blank, although the querystring contains key value pair. This causes the webservice integration piece to fail. What can cause this issue where the querystring value is blank? and how can I fix it? 
http://www.wow.com/Request.aspx?Req_ID=7 
Thank you.
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