Allow array of booleanNodes with random keys with Symfony Configurations and default values

I want to achieve the following configuration structure:

app:
    integrations:
        some: true
        random: true
        options: true
        user_can_make_these_up: true

Where I already added some defaults: mykey: true and myotherkey: false

So in this case the compiled configuration should be:

[
    mykey => true,
    myotherkey => false,
    some => true,
    random => true,
    options => true,
    user_can_make_these_up => true,
]

I tried the following:

$rootNode
    ->children()
        ->arrayNode('integrations')
            ->ignoreExtraKeys()
            ->addDefaultsIfNotSet()
            ->children()
                ->booleanNode('mykey')->defaultTrue()->end()
            ->end()
        ->end()
    ->end();

But then I get an error “Unrecognized option “mykey” under “app”. Available option is “integrations”.”

What am I doing wrong?