Missing argument 1 for Illuminate\Support\Manager::createDriver()

I have a project I'm working on which works absolute fine locally, and fine on my development server.

However, I'm trying to setup a build environment using Gitlab-ci and getting the following error message when trying to send an email.

Missing argument 1 for Illuminate\Support\Manager::createDriver(), called in /root/builds/8103aae9/0/shepparddigital/supafanzv2_api/vendor/illuminate/support/Manager.php on line 87

My build environment grabs the project files from git, runs 'composer install' and then attempts to run tests using PHPUnit. 45 of the 46 tests I have created pass, but one fails. I've tracked it down to a method where i'm trying to send an email;

Mail::send('mails.user-password', $viewData, function ($m) use ($user) { $m->from('test@test.com', 'My Project');

                    $m->to($user->email, $user->full_name)->subject('My Project :: Password reset!');
                });

On my local environment all 46 tests pass. And if I run the tests on my development server they also pass. For some reason the single test doesn't pass during the build process.

I'm wondering if anyone has come across a similar issue?

To confirm, I have a Mail.php file in my config folder which uses values defined in the .env file. As part of my debugging process I've checked the value of the environment properties and things such as the MAIL_HOST etc are all correct so I know these are being loaded.