dovecot - May 2017 - error trying to access the doveadm http api

I?m trying out the the doveadm http api for the first time. When I do:

$ curl -H "Authorization: Basic <base64
doveadm:doveadm_password>" http://localhost:8888
curl: (52) Empty reply from server

In the logs, I see the following error:

May 09 11:58:05 doveadm(10.0.0.17): Error: doveadm client not compatible with
this server (mixed old and new binaries?)

I am running Dovecot 2.2.29.1 in Docker containers.

What am I doing wrong?

Kevin