Python Flask errorhandler unit test code coverage

While trying to improve code coverage of unit tests in my python flask app I am encountering some code coverage gaps.

So there is a 404 error handler

@app.errorhandler(404)
def page_not_found(e):
    app.logger.exception(e)
    if isinstance(e, HTTPException):
        return e
    return "404 Page not Found", 404

With this unit test

def test_404_check(app, client):
    res = client.get('/testof404')
    assert res.status_code == 404

It is passing but there is no code coverage for the errorhandler function.

How should I test this errorhandler in order to have the functions code covered?