Verify a Shopify hook signature using Laravel Middleware

VerifyShopifyWebhook.php

<?php

namespace App\Http\Middleware;

use Closure;

class VerifyShopifyWebhook
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        if (! $this->verifyShopifySignature($request)) {
            abort(401);
        }

        return $next($request);
    }

    private function verifyShopifySignature($request)
    {
        return ($request->header('x-shopify-hmac-sha256') == $this->calculateHmac());
    }

    private function calculateHmac()
    {
        $data = file_get_contents('php://input');
        return base64_encode(hash_hmac('sha256', $data, env('SHOPIFY_APP_SECRET'), true));
    }
}

Hello,

How this works exactly? I can successfully verify the signature but I'm getting a 405 => Method not allowed. I'm using the middleware as a routeMiddleware.
Any help would be appreciated.

Thank you.