Blank page when trying use direct injection with ProductRepositoryInterface class

I am fairly new to Magento and following a tutorial on Udemy to learn how to develop on Magento 2.

I am attempting to overwrite the ProductRepositoryInterface, however whenever I add the preference tag in my di.xml file I am getting a blank page.

When I remove the preference tag my di.xml works fine.

I’ve attached my code below.

I have tried cache:clean and setup:upgrade with no luck.

If somebody is able to stear me in the right direction I’d be very grateful.

Thanks, Pete!

Controller/Page/HelloWorld.php

<?php

namespace PHPeteFirstModuleControllerPage;

use MagentoFrameworkAppActionContext;
use MagentoFrameworkAppResponseInterface;
use PHPeteFirstModuleNotMagentoPencilInterface;
use MagentoCatalogApiProductRepositoryInterface;

class HelloWorld extends MagentoFrameworkAppActionAction {

    protected $pencilInterface;
    protected $productRepository;

    public function __construct(Context $context, PencilInterface $pencilInterface, ProductRepositoryInterface $productRepository){
        $this->pencilInterface = $pencilInterface;
        $this->productRepository = $productRepository;
        parent::__construct($context);
    }

    public function execute(){
        echo 'Hello World<br>';
        echo get_class($this->productRepository);
    }
}

Model/CustomImplementation.php

namespace PHPeteFirstModuleModel;

use MagentoCatalogApiProductRepositoryInterface;

class CustomImplementation implements ProductRepositoryInterface
{

    public function save(MagentoCatalogApiDataProductInterface $product, $saveOptions = false);

    public function get($sku, $editMode = false, $storeId = null, $forceReload = false);

    public function getById($productId, $editMode = false, $storeId = null, $forceReload = false);

    public function delete(MagentoCatalogApiDataProductInterface $product);

    public function deleteById($sku);

    public function getList(MagentoFrameworkApiSearchCriteriaInterface $searchCriteria);
}

etc/frontend/di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="PHPeteFirstModuleNotMagentoPencilInterface" type="PHPeteFirstModuleNotMagentoBigPencils" />
    <preference for="MagentoCatalogApiProductRepositoryInterface" type="PHPeteFirstModuleModelCustomImplementation" />
</config>