TypeError: Argument 3 passed to WHMCS\Domains\AdditionalFields::processFieldOverrides() must be of the type array

Symptom

When viewing a domain registration in the Admin Area or Client Area, you may encounter a TypeError error message:

TypeError: Argument 3 passed to WHMCS\Domains\AdditionalFields::processFieldOverrides() must be of the type array, null given, called in /path/to/whmcs/vendor/whmcs/whmcs-foundation/lib/Domains/AdditionalFields.php

Cause

This error is due to a syntax error in the /resources/domains/additionalfields.php file. You can use this file to customize the additional domain fields for each TLD.

Solution

Review the contents of your /resources/domains/additionalfields.php file for syntax errors like missing quotation marks, commas, parentheses or semicolons.

For information on the correct syntax to see, see the examples in Additional Domain Fields.

After you have identified and corrected the syntax error, upload the corrected additionalfields.php file to your server.