User agent returns null when client is using wcf test client

I want to write information about user agents to a log file. So I am using this code:

System.Web.HttpContext.Current.Request.UserAgent;

      

        
        
        
      

    

It works fine when I use the browser to consume my web service (the user agent returns the correct value), but when I use the WCF test client it returns an empty string. The IIS log user agent returns the correct value regardless of which application is using the web service. I guess I missed something, but now I don't know.