Dataset - Head Sample preview error

[andrew_bates]

Hi we are trying to use a Kafka connection to create a dataset and selecting JSON format:

However when Pipeline Designer tries to create a preview I am getting the following error:

We would like to understand what the "Not in union" error means and what I need to do to resolve it?

[andrew_bates]

Seen https://stackoverflow.com/questions/35963285/org-apache-avro-unresolvedunionexception-not-in-union-long-null but doesn't really help narrow issue down...