I'm trying to enable the Test Cases capability. I'm getting an error: "Please provide a valid base hierarchy!" when trying to save a Test Case record.

I have created a Hierarchy record. But doesn't seem to be a related to the Test Case object. The error message is not coming from a Validation Rule on the Test Case object. It seem to be coded in to some of the hidden Apex logic. Is there a workaround to enable the Test Case object/tab? I'd like to use this as part of the Test Plan and Test Execution across User Stories.