Issue with custom extensible entity and getList method from repository

I have a custom extensible entity:

1. interface that extends ExtensibleDataInterface with get and set;
2. model that extends AbstractExtensibleModel with get and set;

My GetList method from repository:

    /**
     * @param SearchCriteriaInterface $criteria
     * @return AskQuestionSearchResultsInterface
     */
    public function getList(SearchCriteriaInterface $criteria)
    {
        $searchResults = $this->searchResultsFactory->create();
        $searchResults->setSearchCriteria($criteria);

        $collection = $this->askQuestionCollectionFactory->create();

        $this->extensionAttributesJoinProcessor->process(
            $collection,
            AskQuestionInterface::class
        );
        $this->collectionProcessor->process($criteria, $collection);

        $searchResults->setItems($collection->getItems());
        $searchResults->setTotalCount($collection->getSize());
        return $searchResults;
    }

I have no idea what I am doing wrong. When I try to get items from web API endpoint it says that I don’t have the interface class.

"message": "Class "AskQuestionInterface" does not exist",
"code": -1,