Error "Could not load requirements from server" in the Overall Requirement Report

Error "Could not load requirements from server" in the Overall Requirement Report when the value "None" is an option of a custom field (Select List) and the result is grouped by the custom field.

How to reproduce:

1. Create a new customer field of type Select List.
2. Add several options to the field and also add an option "None".
3. Open an Overall Requirement Coverage and group by this new field.
4. A new group with value "None (?????)" should appear and the error is returned when we click on the bar with the percentage of coverage to see the requirements.