I have a product reviews on a site and I am adding schema.org markup to the reviews. Here is the code I am using:

<div class="blockquote-wrap">
 <blockquote itemprop="review" itemscope itemtype="http://schema.org/Review"><span itemprop="reviewBody">Text of the review itself.</span>
<cite><span itemprop="author">Author Name</span>, Location of Author</cite>
</blockquote> 
</div>

This is all the reviews are. When I test the page using Google's Structured Data Testing Tool I receive this error:

Error: Incomplete microdata with schema.org.

My question is what data is missing that is required? I don't see which data is required on the Schema.org page for reviews.