Use subdomain for certain urls

I have the following rules in my .htaccess file on a WordPress (single installation):

# REWRITE FORUMS DOMAIN TO FOLDER
RewriteCond %{HTTP_HOST} ^forums\.example\.com$
RewriteRule !^forums/? forums%{REQUEST_URI} [NC,L]

# REWRITE FORUMS FOLDER TO SUBDOMAIN
RewriteCond %{THE_REQUEST} \s/forums/([^\s]*) [NC]
RewriteRule ^ http://ift.tt/1IpFWfw [R=301,L]

This is so that the forums that exist at http://ift.tt/1jBarQy are accessed at http://ift.tt/11ksZ6O

However I get a server error... Any ideas on how to do this? These rules work perfectly when used on non-wordpress sites...