I am trying use a Nginx map assigned variable in an upstream but it doesn't seem to work?

The map is concatenated $uri$args assigning a PHP-FPM fastcgi PHP pool to variable $pool and then setting the $pool variable in an upstream.

map $uri$args $pool {
default 127.0.0.1:9000;
"~/index.php/args" 127.0.0.1:9002;
}

upstream php {
zone php_zone 64k;
server $pool;
keepalive 2;
}

But if I try this, nginx config test gives me

nginx -t
nginx: [emerg] host not found in upstream "$pool" in ...

What am I missing?

cheers

George