Security - Using a Custom Query to Load the User does not work

Symfony 3.0
PHP 5.6.11
Mac OS
PHP Built in webserver

I am following this part of the documentation:

to enable authentication with a custom query.

So I remove the 'property' entry at 'security.yml' file. And I take this message when I try to login:

"Authentication request could not be processed due to a system problem."

Same code worked for me in symfony 2.7.7. Same Entity, same entity repository ...

If I use 'property: username' it works.

This works:

providers:
    db_users:
        entity:
            class: AppBundle:User
            property: username

This does not work:

providers:
    db_users:
        entity:
            class: AppBundle:User

My code:

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