Using all_databases: true requests parameter called "database_name"

For some reason when using all_databases: true it requests a parameter called %database_name%. Under normal Symfony installations this parameter exists, but in my case I have an alternative configuration that does not define this parameter.

Surely when using all_databases: true then no database name parameter should be required at all?

Temporary solution is simply to define the parameter:

parameters:
  database_name: ~

I'm using Symfony 3.3 with Doctrine / MySQL