nginx load different files if varibles

I want to server up landing.html if they come without any url parameters and send them to app.html if they do. I would think this would work but it tells me I can't try files within that block.

server {
        root /home/username/client/projects/web-apps/public;
        index landing.html index.html index.htm;

        # Make site accessible from http://test1.com/
        server_name test1.com;

        location ~ ^/(\d\d\d\d\d) {
                try_files $uri $uri/ /app.html;
        }

        location /q {
                try_files $uri $uri/ /app.html;
        }

        location /  {
                set $page 'landing';
                if ($args) {
                        set $page 'app';
                }
                try_files $uri $uri/ $page.html;
        }

        location /ad {
                try_files $uri $uri/ /app.html;
        }

        location /Location {
                try_files $uri $uri/ /landing.html;
        }  
}