Problem with adding custom formatter nelmio alice

Question

I'm trying to add custom formatter in symfony3.4. My code looks like this: https://stackoverflow.com/a/49609522/6505678

Looks like smth is wrong with my CustomFixtureProvider, but I can't grasp what it is. I think it shouldn't implement Doctrine\Common\DataFixtures\FixtureInterface, because it requires method load(), and in CustomFixtureProvider there's shouldn't be such method.

Yes, there is a question like this but this one is about specific issue with adding formatter.

PHP TypeError:  Argument 1 passed to Doctrine\Bundle\FixturesBundle\Loader\SymfonyFixturesLoader::addFixture() 
must implement interface Doctrine\Common\DataFixtures\FixtureInterface, 
instance of AppBundle\DataFixtures\CustomFixtureProvider given, 
called in /var/www/tut/vendor/doctrine/doctrine-fixtures-bundle/Loader/SymfonyFixturesLoader.php 
on line 36 in /var/www/tut/vendor/doctrine/doctrine-fixtures-bundle/Loader/SymfonyFixturesLoader.php on line 40