Can't query products slug on a given collection

[nonlinearcom]

Describe the bug
on this repoduction setup it's not possible to get a list of products-slug from a given collection:

{
	collection(slug: "collection1") {
		id
		productVariants {
			items {
				product {
					id
 					slug
				}
			}
		}
	}
}

we get this error: Cannot convert undefined or null to object but if we comment the slug bit, thequery works normally

To Reproduce
Steps to reproduce the behavior:

1. Clean Install vendure 2.0.2 without dummy data.
2. Add dummy products, facets, colletcions and channels.
3. Add custom fields (here is where the query gets broken)

https://github.com/nonlinearcom/vendure-reproduction

Expected behavior
if we query a collection, we should be able to get a list of products with their slugs :)