Products are not retrieved from Sitecore Ecommerce Services

Question

I'm trying to integrate Sitecore E-Commerce Services (SES) into an existing site with existing products (available as Sitecore items).

When I try to retrieve the products like this:

 IProductRepository productRepository = Sitecore.Ecommerce.Context.Entity.Resolve<IProductRepository>();
 var builder = new CatalogQueryBuilder();
 var productTemplateId = "{C7B33629-A35B-4EF8-8FE7-9343ACAA6FBE}";
 var productTemplate = new ListString { productTemplateId };
 var query = builder.BuildQuery(new SearchOptions { Templates = productTemplate });
 var products = productRepository.Get<ProductBaseData, Query>(query);

The products variable is empty and no exceptions are thrown.

Any ideas what I might be missing?

Info about templates & config

The product items use a custom template which derives from the ProductBaseData template. The product repository item derives from the Repository template.

The following is added to the Unity config:

<alias alias="MyProduct" type="MyNamespace.MyProject.Product, MyNamespace.MyProject" />

<register type="ProductBaseData" mapTo="MyProduct" name="{C7B33629-A35B-4EF8-8FE7-9343ACAA6FBE}"/>

In addition the Lucene index has been configured to point to the product repository and I can see (using Luke) that the products are being indexed.