No Application Encryption Key Has Been Specified

I'm trying to use the `Artisan` command like this: php artisan serve It displays: > Laravel development server started: <http://127.0.0.1:8000> However, it won't automatically launch and when I manually enter http://127.0.0.1:8000 it shows this error: > RuntimeException No application encryption key has been specified. What's the cause of this problem, and how can it be fixed? I'm using **Laravel framework 5.5-dev**.