add_editor_style is not working

I've reviewed various blog posts, stackexchange topics, etc, but can find no way to get add_editor_style() to work in WordPress 4.1.5.

Is anyone having any luck getting this function to work in recent versions of WordPress? Can I see how you've done it?

Currently I have something like the following in my functions.php file:

add_action( 'init', 'my_theme_add_editor_styles' );
function my_theme_add_editor_styles() {
  add_editor_style('editor-style.css');
}

I see my editor-style.css referenced in the tinyMCEPreInit settings in the head (among the stylesheets assigned to 'content_css'), but there is otherwise no reference to editor-style.css in the head of the document.

PS -- The following also does not work:

function my_theme_add_editor_styles() {
  add_editor_style('editor-style.css');
}
add_action('admin_init', 'my_theme_add_editor_styles' );