Custom properties on Create Page causes 404

<?php

namespace App\Filament\Resources\Content\ChapterResource\Pages;

use App\Filament\Resources\Content\ChapterResource;
use App\Models\Content\Series;
use Filament\Actions;
use Filament\Resources\Pages\CreateRecord;
use Illuminate\Routing\Exceptions\UrlGenerationException;

class CreateChapter extends CreateRecord
{
    protected static string $resource = ChapterResource::class;

    // THE FOLLOWING LINE GIVES 404 ERROR, WHEN REMOVED THE PAGE LOADS
    public ?Series $series = null;

    public function mount(): void
    {
        $series_id = request()->route('series');

        if (!$series_id || !is_numeric($series_id))
            throw new UrlGenerationException('Invalid series provided!');

        dd($series_id);

        $this->series = Series::findOrFail($series_id);

        parent::mount();
    }

    /**
     * {@inheritdoc}
     */
    public function getBreadcrumbs(): array
    {
        return [];
    }

    protected function mutateFormDataBeforeCreate(array $data): array
    {
        dd($this->series);

        $data['series_id'] = $this->series->id;

        return $data;
    }
}