Or were you not embedding this within another type?
Since "Every web page is implicitly assumed to be declared to be of
type WebPage,"  could you use itemprop="reviews"? Would it work
I find throughout schema.org that I'm always having to check whether a
property ought to be plural or singular. For instance for WebPage the
property is "comment" and then "reviews" even though it seems that
both of these would often be lists or repeated.