Set PIHOLE_DNS_ to docker service in docker-compose fails

hiya,
The docs say you can set this variable to docker service, eg upstream0;upstream1 where upstream0/1 are docker services. Setting the value to a docker service that is working in a docker-compose file yields the following error. Have tried a few permutations, what am I missing please? Have used the "Expose" directive in the dnscrypt-proxy service to expose port 5353:

config:
environment:
PIHOLE_DNS_: 'dnscrypt-proxy#5353' <have tried with and without port
...
dnscrypt-proxy:
image: dnscrypt-proxy-docker:testing
expose: ["5353"]

output:
pihole | Setting DNS servers based on PIHOLE_DNS_ variable
pihole | Invalid IP detected in PIHOLE_DNS_: dnscrypt-proxy#5353
pihole | No Valid IPs dectected in PIHOLE_DNS_. Aborting
pihole | [cont-init.d] 20-start.sh: exited 1.

running a shell inside the pihole container, can resolve the dnscrypt-proxy address to the other container so im wondering if this variable does actually support the service as suggested in the documentation?

Likely unrelated to your issue, but 5353 is reserved for mDNS usage - you should switch to another port.

Are both containers hosted by the same Docker daemon?
Also, please share your respective docker-compose files.

Was a simple logic error in the container order. Looking at start.sh in the pihole code it confirms its just a dig to see if the value resolves. Destination service for PIHOLE_DNS_ hadn't registered itself in time for the valid ip check so pihole failed, so just changed the depends on order and it works fine now. thanks for reading

thanks mate