Symfony: Custom Form Type throws "This value is not valid" error despite having the correct value type

Question

I am creating a web app via Symfony and EasyAdmin. Within a form, I created a custom form field to render an integer. The integer is a mapped value of the form entity.

The form field:

$laufzeit = MonateChoiceField::new('laufzeitMonate');

The custom form type:

$builder
        ->add('laufzeit_choice', ChoiceType::class, [
            'choices' => [
                '24 Monate' => 24,
                '36 Monate' => 36,
                '60 Monate' => 60,
                'Unbefristet' => -1,
                'Eigene Eingabe' => -2,
            ],
            'label' => 'Laufzeit in Monaten'
        ])
        ->add('laufzeit_custom', IntegerType::class, [
            'attr' => [
                'class' => 'form-control ',
                'min' => '0',
            ],
            'invalid_message' => 'custom',
            'label' => 'Benutzerdefinierte Laufzeit in Monaten',
        ])
    ;

Basically, I want my value to load into the ChoiceType if it is within the given choices, and into the IntegerType if it isn't.

My event listener for PRE_SET works perfectly fine:

$data = $event->getData();
    $choiceData = -2;
    switch ($data) {
        case 24:
        case 36:
        case 60:
        case -1:
            $choiceData = $data;
            break;
        default:
            break;
    }
    $dataNew = [
        "laufzeit_choice" => $choiceData,
        "laufzeit_custom" => $data,
    ];
    $event->setData($dataNew);

However, when trying to save the form, I get the error message "This value is not valid." with no further details. My PRE_SUBMIT looks as follows:

$data = $event->getData();
$dataNew = intval($data['laufzeit_choice']);
if($data['laufzeit_choice'] == -2) {
    $dataNew = intval($data['laufzeit_custom']);
}
$event->setData($dataNew);

So my submitted data is an integer, either the value of the ChoiceType or the IntegerType. So in theory the form gets the correct format.

What am I missing?