Html5 notifications: registration failed

Configuration
1

After setting up html5 notifications as described in the docs, I get the following error when trying to activate it through the profile page:

Registration failed - missing applicationServerKey, and gcm_sender_id not found in manifest

notify:
   - platform: html5
     name: html
     vapid_prv_key: <redacted>
     vapid_pub_key: <redacted>
     vapid_email: <redacted>

I’m running HA behind an nginx proxy and I’ve added the path as specified in the docs. I can’t see any errors in the nginx access or errors logs either.