Custom Property on Folders Only Doesn't show as a column option in reports

I would like to have a custom property that only applies to folders. If I do this, the custom property isn't showing up as a column option in the reports. I have to have it included on something other than folders to get the column to be selectable on the reports page.

If I apply it to something else as well like item variables, it becomes a column I can choose in the reports.