Livewire form, validation problem

In my form, name validates correct, but not amount. If I type in a string, like abc into the forms amount field, a 0 is inserted into the database. I expect a validation error to be thrown instead.

<?php

namespace App\Livewire\Forms;

use App\Models\Invoice;
use Livewire\Form;
use Illuminate\Validation\Rule;

class InvoiceForm extends Form
{
    public ?Invoice $invoice = null;

    public string $name = '';
    public $amount = null;

    public function setInvoice(?Invoice $invoice = null): void
    {
        $this->invoice = $invoice;
        $this->name = $invoice->name;
        $this->amount = $invoice->amount;
    }

    public function save(): void
    {
        $validated = $this->validate();
        $amount = $validated['amount'] ? (int) $validated['amount'] : null;

        if (empty($this->invoice)) {
            Invoice::create(array_merge($this->only(['name']), ['amount' => $amount]));
        } else {
            $this->invoice->update(array_merge($this->only(['name']), ['amount' => $amount]));
        }

        $this->reset();
    }

    public function rules(): array
    {
        return [
            'name'        => [
                'required',
                Rule::unique('invoices', 'name')->ignore($this->component->invoice),
            ],
            'amount' => [
                ['nullable', 'integer']
            ],
        ];
    }

    public function validationAttributes(): array
    {
        return [
            'name' => 'name',
            'amount' => 'amount',
        ];
    }
}

<div class="p-6">
    <form wire:submit="save">
        <div>
            <x-input-label for="name" :value="__('Name')" />
            <x-text-input wire:model="form.name" id="name" class="mt-1 block w-full" type="text" />
            <x-input-error :messages="$errors->get('form.name')" class="mt-2" />
        </div>

        <div class="mt-4">
            <x-input-label for="amount" :value="__('Amount')" />
            <x-text-input wire:model="form.amount" id="amount" class="mt-1 block w-full" type="text"/>
            <x-input-error :messages="$errors->get('form.amount')" class="mt-2" />
        </div>

        <div class="mt-4">
            <x-primary-button>
                {{ __('Save') }}
            </x-primary-button>
        </div>
    </form>
</div>

<?php

namespace App\Livewire;

use App\Models\Invoice;
use Illuminate\Contracts\View\View;
use LivewireUI\Modal\ModalComponent;

class InvoiceModal extends ModalComponent
{
    public ?Invoice $invoice = null;
    public Forms\InvoiceForm $form;

    public function mount(Invoice $invoice = null): void
    {
        if ($invoice->exists) {
            $this->form->setInvoice($invoice);
        }
    }

    public function save(): void
    {
        $this->form->save();

        $this->closeModal();

        $this->dispatch('refresh-list');
    }

    public function render(): View
    {
        return view('livewire.invoice-modal');
    }
}