Nikon D6 Reference Manual page 1064

Alerts and Error Messages
This section lists the alerts and error messages that appear in
the viewfinder, top control panel, and monitor.
Alerts
The following alerts appear in the top control panel and
viewfinder:
Alert
Top
control Viewfinder
panel
B
(flashes)
H
1012 Alerts and Error Messages
Lens aperture ring is not set to minimum
B
aperture.
(flashes) Set lens aperture ring to minimum aperture
(highest f-number).
Low battery.
d
Ready spare battery.
Problem/solution