While checking for accessibility using the WAVE chrome extension, I have noticed that I am getting accessibility errors for the navigation & setting blocks tree nodes. (see attached screen capture)

"Broken ARIA reference - An aria-labelledby or aria-describedby reference exists, but the target for the reference does not exist."

For the following piece of code:

<li class="type_setting depth_2 item_with_icon" tabindex="-1" aria-labelledby="label_2_1">

<p class="tree_item hasicon tree_item leaf" role="treeitem" tabindex="-1" aria-selected="false">

<a href="http://localhost/moodle-org/master/course/view.php?id=10&sesskey=vATSxuT0Iw&edit=on" tabindex="-1">

<img class="smallicon navicon" alt="" src="http://localhost/moodle-org/master/theme/image.php/more/core/1475581667/i/edit" tabindex="-1">

Turn editing on

</a>

</p>

</li>

Error is referred to: aria-labelledby="label_2_1"

I have tested the above "what seems like an error" with ChromeVox 53.0.2784.0 screen reader and it reads the tree nodes just fine, although the missing aria-labelledby ids.
So I am wondering if "aria-labelledby" is redundant? or that other screen readers (JAWS, NVDA, VoiceOver, ...) consider it as a real error and does not read the text properly?