.htaccess issues with CI3 inside Wordpress directory

I usually just use a single .htaccess:

Code:

# URLs routed to CodeIgniter
RewriteRule ^foo/.*$ /ci_index.php [L]
RewriteRule ^bar.*$ /ci_index.php [L]

# Everything else goes to WordPress
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule .* index.php/$0 [PT,L]
 

Unfortunately this throws a 500 error on my CI pages.