Why limit_conn is not allowed in such a configuration?

#http context

limit_zone resp_zone $binary_remote_addr 64m;
geo $limit_conn {
default 1;
x.x.x.x/32 0;
z.z.z.z/32 0;
y.y.y.y/32 0;
}

#server context
server {
listen 80;

if ($limit_conn) {
limit_conn resp_zone 5;
}

location / { proxy_pass http://127.0.0.1:8080; }

error_page 500 501 502 503 504 505 /50x.html;
location /50x.html {
#root /vhosts/overload;
root /vhosts/maintenance;
}

include base/status;
}