Date format works with n1ql but not analytics

I have a document that has a datetime of format "2019-01-16T16:56:31.235254Z", and a query like:

SELECT DATE_PART_STR(start_time, "month") FROM default LIMIT 15

When running against n1ql this returns me a list of documents of the form 

{{{}}
{{ "$1": 1}}
{{ },}}

However, when I run against analytics it does not appear to handle the format correctly and I get a list of documents that look like
{{ {}}
{{ "$1": null}}
{{ },}}