RewriteMap rnd in .htaccess symfony2

Question

I'm currently trying to do a simple random load balancing using 2 servers with symfony2.

In the /web .htaccess file, I have tried a lot of combinations, but result is always blank and I do not see a way to debug. What could help ?

.htaccess file :

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteMap servers rnd:/map.txt

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ app.php [QSA,PT]
</IfModule>

The map.txt is in the /web folder too. The problem occurs when I add the line RewriteMap servers rnd:/map.txt