Drupal - Missing bundle property on entity of type comment

After 24 hours of fudging around, I finally figured out what causes this problem. To make a long story short, I needed to make sure the node_type property was set on my comment entity. For example, a comment for an article node needs this property set:

$entity->node_type = 'comment_node_article';

The error message itself is a little misleading, since it says the bundle property is missing. But in actuality, it was looking for the node_type property to be set also.

Background

I am using the Comment Index Resource from the Services module. During hook_services_request_postprocess_alter() I am attaching comment fields to each result, since they don't come packaged with comment index results by default. To do this, I was using the field_attach_load() function, which requires entity(ies) be passed to it. I was setting the bundle property on the comment entities, since I am so used to having to set that property. But apparently the node_type property is really what needed to be set for comment entities.

Strange indeed. Gotta love those one liners, just like the ladies.