cookies - nginx can't proxy_pass request using map cookie

Question

I want to use cookie to indicate which backend server to perform the request. here is my configuration:

upstream backend_prod   {
    server 127.0.0.1:8080;
}
upstream backend_abenv   {
    server 127.0.0.1:9090;
}
map $COOKIE_backend $gated {
    default prod;
    eexxpptt abenv;
}
server {
    listen       8888;
    server_name  localhost;
    location / {
        index  index.jsp index.html index.htm;
        proxy_pass http://backend_$gated;
    }
}

I have set cookie backend:eexxpptt. But I got a weird URL http://backend_prod when I visit http://localhost:8888 in chrome. And do not reach the 8080 or 9090 server.

Any advice?