comment_text filter simply not working.

I’ve got a plugin with several filters: the_content, the_excerpt, and newly added, comment_text. I’ve been scratching my head trying to figure out why the comment_text filter simply does not get applied.

The add_filter call:

add_filter('comment_text', 'ref_process_comment', 5);

The filter:

function ref_process_comment($text) {
   return "This space intentionally left blank.";
}

It doesn’t do anything. I’ve been disabling and re-enabling the plugin, even deleting and reinstalling it. The other filters work fine, but not this one.

What am I missing?