When the user clicks the "review your registration button" the confirmation screen gives a warning.
Warning: array_key_exists(): The first argument should be either a string or an integer in /home/customer/www/crm.coma.org/public_html/wp-content/plugins/civicrm/civicrm/CRM/Utils/Array.php on line 39
Everything works if they click "continue" but we think this message may be putting off people from registering.
I don't really know when this started, but the only thing I have changed recently is to install Cividiscount. I'm running CiviCRM 5.35.1 on WordPress. The version of Cividiscount that installed was 3.8.2 which should be OK. Disabling Cividiscount doesn't help.