error in validator reporting html comments incorrectly

embedding an html comment, <!-- comment --> in a document, using the 
html5 validator reports an error, perhaps because it considers the <! of 
the doctype as a start comment.  The error it reports is:
--!> found at end of comment (should just be -->).

This is validator version 17.5.19  This error wasn't being reported even 
a couple of days ago.

Thanks, Rob