Content-Length header missing

Running Drupal 8 on php 7.2 and nginx 1.14.0.

We are using cloudfront and to utilise the built in compression in CF it requires that we send a Content-Length header.

If I turn gzip off; on nginx we don't get a Transfer-Encoding: chunked header which I understand you can't have both Transfer-Encoding & Content-Length but we now get neither.

Is there a way to get nginx to add the header?

Thanks

Jon