GraphQL: Cannot return null for non-nullable field

I have been testing the GraphQL API to retrieve the proposals from the PAM process at Decidim Barcelona. By using a basic query to get id, title and author name (see http://bit.ly/2LkKKO0) some proposals of the first page are shown as null with an error message saying "Cannot return null for non-nullable field Proposal.author". This happens in proposals like BCN-PROP-2016-01-966 with null 'author' while this field was defined as a non-nullable (this error also exists in other fields like 'scope').