That assembly does not allow partially trusted callers

I am trying to use a winform user control in an .aspx page. I could do it successfully without any issue. But now when I am trying to use an external .dll inside this user control then it is throwing me an error – "That assembly does not allow partially trusted callers". But I could use this same user control inside a winform without any error. How could I make my web application a fully trusted to an external .dll? Kindly help.