Curious Fluke 8846A Behavior (bug?)

[kwass]

I was setting up to use the rear terminals on my Fluke 8846A and saw something buggy.

When I put it in the capacitance mode, rear terminals selected with no leads plugged in, it read a few pf of negative capacitance.  Once I plugged in leads, all was fine and I got accurate results.  I've never seen negative capacitance numbers using the front terminals.
FYI: OutG=2.31.926.24, InG=ad300220

As Bob Pease would have said,  "What's All This Negative Capacitance Stuff"?