session kontra cookies

Hvorfor kan jeg ikke dette:

<%

skin_number = request.querystring("skin_number")

if skin_number = "" AND Request.Cookies("skin_number") = "" then response.cookies("skin_number") = ""
elseif skin_number <> "" then
    response.cookies("skin_number") = skin_number
end if

%>

men dette:
<%

skin_number = request.querystring("skin_number")

if skin_number = "" AND session("skin_number") = "" then session("skin_number") = ""
elseif skin_number <> "" then
    session("skin_number") = skin_number
end if

%>