ISPConfig 3.1 under nginx: nginx_plugin.inc.php missing in_array() on line 748

When installing a fresh copy of ISPConfig 3.1, /usr/local/ispconfig/server/plugins-available/nginx_plugin.inc.php fails on line 748 with 'unexpected comma'.

Closely inspecting the equivalent snippet of code on the Apache2 plugin, it seems that line 748 ought to read:

if ( in_array($file_system , array('ext2','ext3','ext4'), true) ) {

instead of

if ( $file_system , array('ext2','ext3','ext4') ) ) {

which fails.