Nikon D90 + Sigma EF-500 DG Super. Camera settings blank?

I just got a new flash, and for some reason if i put the flash on the TTL setting the top camera screen goes blank - the apperture and the shutter speed disappear… What am i doing wrong?

Added (1). The 'Flash Mode' Square symbol thing flashes… And also if i switch on the back screen of the camera to see settings it has the flash mode box flashing… With TTL inside to the right of it… There must be a way to fix this…

Is it because on TTL the cameras shutter speed is being set by the flash? I don't know 100% as I don't have a Nikon but in Av mode when using my flash on P-TTL the shutter speed is fixed. Obviously I can override this depending on the setting/mode used.

I have seen this before with many other camera flash and lens types and configurations as its a sigma flash and you are metering through the lens it is likley you will need a sigma lens to get it all to work properly or buy a nikon SB type flash they are excellent and start at only $200 cheap option for now is not to use ttl as its normally to bright any way stick it on A mode or GN it will still meter the light output duration and work just fine only in M mode you have to check your distances and iso and what not

I don't think you're doing anything wrong, I think there's something wrong with that flash. I've seen this combo before and a blank top lcd is NOT normal