Null follower emails

I’m hitting https:\/\/${campaign_id}/members?include=currently_entitled_tiers&fields%5Bmember%5D=email

Get get my Patrons and Follower’s email addresses so that I can unlock functionality for them in my application.

I am getting the email addresses of my patrons fine but for followers I’m only getting null. Is this expected behaviour or is there something I can change to fix this issue?