Multiple dynamic dns services fails with "Invalid Value for keyword 'ip' = ''"

I am trying to run two dynamic DNS services. One is zoneedit dynamic DNS; the other is HE Tunnel Broker, which uses dyndns2 protocol but isn't actually a DNS service.

Here is my configuration:

 show service dns dynamic
 interface pppoe0 {
     service custom-he {
         host-name tunnel204625.tunnelbroker.net
         login xxxxx
         password xxxxx
         protocol dyndns2
         server ipv4.tunnelbroker.net
     }
     service zoneedit {
         host-name home.tomh.us
         login xxxxx
         password xxxxx
     }
 }

Now when I view the log I see this error and the second service is not updated:

Nov  9 12:16:42 ubnt ddclient[4039]: WARNING:  file /var/cache/ddclient/ddclient_pppoe0.cache, line 3: Invalid Value for keyword 'ip' = ''
Nov  9 12:16:42 ubnt ddclient[4039]: WARNING:  skipping update of home.tomh.us from <nothing> to 97.113.234.110.
Nov  9 12:16:42 ubnt ddclient[4039]: WARNING:   last updated <never> but last attempt on Wed Nov  9 12:15:40 2016 failed.
Nov  9 12:16:42 ubnt ddclient[4039]: WARNING:   Wait at least 5 minutes between update attempts.

If I delete either of the two services, the remaining one works fine. This tells me there are no issues with passwords or protocols.

delete service dns dynamic interface pppoe0 service custom-he

Nov  9 12:22:14 ubnt ddclient[4506]: SUCCESS:  updating home.tomh.us: IP address set to 97.113.234.110 (201: no update required for home.tomh.us to 97.113.234.110)

I'm running v1.9.0.

I see this error in other posts, but of the ones I read (which I think were all of them), they are all related to incorrect passwords, protocols, old software that didn't support multiple ddns services, etc. As far as I can tell, none of those apply here. Like I said, deleting either of the two services causes the remaining one to start working, so there must not be an error with each of the service definitions themselves.

What can I do to get both dynamic dns services running at the same time?