Is it possible to getRelatedRecords for custom modules?

I can't find anywhere in the documentation that states this doesn't work however I always get the response "Unable to populate data, please check if mandatory value is entered correctly.". Here is an example API call:

https://crm.zoho.com/crm/private/xml/CustomModule2/getRelatedRecords?authtoken={token}&scope=crmapi&parentModule=Contacts&id=1398203000000086138

If I change CustomModule2 in the URL above to something like Notes then it works correctly. I can getRecordById for CustomModule2 and it is definitely linked to Contacts. I can also use getRelatedRecords in custom functions correctly (see attached screenshot for how it is setup). Just not working via the API.

Thanks for any help.