Hi Rob and Welcome to Nikonians.don´t worry about it. It´s normal and correct.It shows the underexposure, which is correct. This underexposure is now correct by the speedlight when you press the shutter-release.Edited to add some more info.
Gerold - Nikonian in East FrisiaEala Freya Fresena
Copyright © Nikonians 2000, 2013
All Rights Reserved
Nikonians®, NikoScope® and NikoniansAcademy™ are trademarks owned by Nikonians.org.
Nikon®, Nikonos® and Nikkor® are registered trademarks of Nikon Corporation.