Fatal error when a 404 occurs

If I try and visit any non-existent URL on my website, the 301-redirects plugin causes a fatal error:

AH01071: Got error 'PHP message: PHP Fatal error:  Uncaught Error: Call to undefined function getallheaders() in /var/www/vhosts/da-media.co.uk/httpdocs/wp-content/plugins/301-redirects/libs/utility.php:331\n
Stack trace:\n
#0 /var/www/vhosts/da-media.co.uk/httpdocs/wp-content/plugins/301-redirects/libs/functions.php(150): WF301_utility::parse_user_agent_array()\n
#1 /var/www/vhosts/da-media.co.uk/httpdocs/wp-content/plugins/301-redirects/libs/functions.php(127): WF301_functions::autoredirect_404('/somepath/')\n
#2 /var/www/vhosts/da-media.co.uk/httpdocs/wp-includes/class-wp-hook.php(287): WF301_functions::redirect('')\n
#3 /var/www/vhosts/da-media.co.uk/httpdocs/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array)\n
#4 /var/www/vhosts/da-media.co.uk/httpdocs/wp-includes/plugin.php(478): WP_Hook->do_action(Array)\n
#5 /var/www/vhosts/da-media.co.uk/httpdocs/wp-includes/template-loader.php(13): do_action('template_redire...')\n
#6 /var/www/vhosts/da-media.co.uk/httpdocs/wp-blog-header.php(19): require_once('/var/www/vhosts...\n'