Error about Missing key (mre): org.sakaiproject.tool.assessment.bundle.AuthorMessages mcms_whats_this_note

When you edit any question in a pool, you can see a link called "What's This?" next to Answer. This is seen in the Multiple Question type.
When this link is clicked, it brings a separate window explaining the difference between simple question, multiple questions, ...etc. On the bottom of the first section which is Simple Question, there is an error about some missing text.

Here's the url of this page: http://nightly2.sakaiproject.org:8081/samigo-app/jsf/author/mcWhatsThis.faces

and here's the text about the error, it is written between brackets:
[missing key (mre): org.sakaiproject.tool.assessment.bundle.AuthorMessages mcms_whats_this_note]