If you receive an error message that says NULL when attempting to apply a discount code, it means that another plugin, script, or configuration on your site is blocking Easy Digital Download’s ability to talk to WordPress through the admin-ajax.php.
This is typically a side effect of using a plugin on the site that blocks access to the /wp-admin/ area of WordPress. To resolve the NULL error, start by deactivating all other plugins on your site and then test applying a discount code. If that resolves it, reactivate the plugins one at a time until the problem comes back. Once it has returned, you will know which plugin is causing it.
If you need to remove access to the /wp-admin/ area, we recommend using the Remove Dashboard Access plugin.