REST API, `Value cannot be null.\r\nParameter name: criteria` trying to get a Workflow

Hello everyone.  I'm using Aras 12SP09 and I'm working with the REST API.

I have an AML call that I would like to transform to a REST call, it is:

<Item type='Workflow' action='get' select='related_id(*)'>
  <related_id condition='is not null'/>

I turned it into the following REST call: .../server/odata/Workflow?$filter=source_id  eq 'UUID' but then I have an error "Value cannot be null.\r\nParameter name: criteria". What I don't understand is, the same syntax is working perfectly fine when trying to get a Property for example: .../server/odata/Property?$filter=source_id eq 'UUID'. Is there something different for workflows? What am I doing wrong?