is_active_sidebar fatal error

Hi there, since upgrading I’m getting a Fatal error:
Fatal error: Call to undefined function is_active_sidebar() in /var/www/vhosts/perspectivaconsultants.com/httpdocs/wp-content/themes/pinboard/header.php on line 23

Any ideas – it’s a single page site just now with very few plugins added (I’ve tested with all turned off.)

Thanks
John