Laravel 5.1 ignores any values inside env()

I am uisn glravel 5.1 and setting up mail service with Mailgun. I've just found that my services file contains lines like the following:

'mailgun' => [
    'domain' => env('<domain>'),
    'secret' => env('<key>'),
],

Now for some reason, these values get ignored as-is. However, if I remove the env() method from the above, it works. So now I have this:

'mailgun' => [
    'domain' => '<domain>',
    'secret' => '<key>',
],

Can anyone explain why this is?

via Chebli Mohamed