Should be sortable for Formatter "Node Count" in view

Steps:
1. Add a field "Node: Nodereferrer: by field" (or by node type) in view
2. Select "Formatter" as "Node Count"
The result is correct but isn't sortable. It is necessary to be sortable for "Node Count" in view, I think.

BTW, "Display as" contains "List items" & "Comma separated list". Maybe need another choice for "Node Count"?