Introduction
This error usually occurs after submitting a form from the front-end of the website that was embedded using a plugin shortcode or widget, such as Contact Form 7 or Jetpack’s Contact Form.
The Solution
- If this error is occuring in a form built using a plugin (Jetpack, Gravity Forms, Contact Form 7 etc). make sure the plugin is configured with the correct email address and form validation data.
- If this error is occuring on checkout in WooCommerce, see Fatal Error or Warning: array_merge(): Argument #1 is not an array
- If this error is occuring on a custom form or in a child theme and the form is trying to submit post data, you must ensure the form is accounting for any required custom field keys. To see which custom fields are assigned to the post type, enable the Custom Fields module on the post editor under Screen Options. Also refer to the theme’s post_type_meta.php file and custom.php to understand how the data is gatered, merged and then processed.
