The solution for me was:
Edit your product, Additional Info, Add code block, do not display source code and add the following code:
<div itemscope itemtype="http://schema.org/Product">
<meta itemprop="productID" content="XX">
<meta itemprop="sku" content="XX">
<meta itemprop="category" content="YYYY">
</div>
where XX is the SKU (identifier of your item) and YYYY is the number of the product you have from the google product catalog at https://www.google.com/basepages/producttype/taxonomy-with-ids.en-US.txt
