Symfony, update 3.4 => 4.4. Monolog error

I am trying to update a simple web application from Symfony 3.4 to Symfony 4.4.

Eventually I get the following error

>php bin/console debug:configIn ScalarNode.php line 36:Invalid type for path "monolog.handlers.main.channels.elements.0". Expected scalar, but got object. 

I am not sure what that means. Is there some new way to configure Monolog?