Unable to add classes to "more" link with filter

I’m trying to add classes to the “more” link but the following filter is not being applied when added in functions.php

function my_add_morelink_class( $link )
{
    return str_replace(
        'more-link',
        'more-link right extra-class',
        $link
    );
}
add_filter( 'the_content_more_link', 'my_add_morelink_class', 10, 2 );