EasyAdminBundle entity form fields customization

I am trying to make a standard "created_at" field readonly in the edit form. Following the doc you have to add the following configuration:

    MyEntity:
        form:
            fields:
                - { property: 'created_at', type_options: { widget: 'single_text' } }

But it throws the following error:

An Exception was thrown while handling: The option "widget" does not exist. Defined options are: "action", "allow_extra_fields"...

Is there something obvious to add/modify ?