Type definition was missing. As the official Terraform documentation states:
> The resource block defines a resource that exists within the infrastructure. A resource might be a physical component such as an EC2 instance, or it can be a logical resource such as a Heroku application.
From: https://www.terraform.io/intro/getting-started/build.html#configuration
Michal Wasilewski
Ryota Kayanuma
Sergey Timanin
Andrea Crotti