The problem was that the custom object was outside of the frequency object; I've fixed it in the experiment that is now in preview mode. It's possible that having the schema object on experimenter and stage will help catch issues like this, and Andrei and I are planning on tightening up the schema to catch more issues as well. I'll be sure that we consider this too, if it's not already caught by the existing schema.
Bug 1756152 Comment 2 Edit History
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The problem was that the custom object was outside of the frequency object; I've fixed it in the experiment that is now in preview mode. It's possible that having the schema set-up on experimenter and stage will help catch issues like this, and Andrei and I are planning on tightening up the schema to catch more issues as well. I'll be sure that we consider this too, if it's not already caught by the existing schema.