Error checking for NULL in VBScript, getting "Object Required"

I have the following VBScript in a Classic ASP page:

function getMagicLink(fromWhere, provider)
    dim url 
    url = "magic.asp?fromwhere=" & fromWhere
    If Not provider is Nothing Then ' Error occurs here
        url = url & "&provider=" & provider 
    End if
    getMagicLink = "<a target='_blank' href='" & url & "'>" & number & "</a>"
end function

I keep getting an "Object Required" error messager on the line that says If Not provider Is Nothing Then.

Either the value is NULL, or it's not NULL, so why am I getting this error?

Edit: When I invoke the object, I pass in either NULL, or I pass in a string.