Hustle Campaign Monitor opt-in doesn't work

I have Hustle pro set up with the shortcode placed in footer (also tested on page) - it is linked to Campaign Monitor, but when I try and submit a sign-up it gives the error 'Something went wrong, please try again
I see the following error in debug.log
[01-Mar-2018 10:19:28 UTC] PHP Fatal error: Call to undefined method Hustle_Module_Model::get_custom_field() in .../wp-content/plugins/hustle/inc/providers/opt-in-campaignmonitor.php on line 125