"Developed For Use Only With NTSC Genesis Systems"

Greetings I've recently installed dgen Sega emulator for on my Ubuntu computer, using TKDgen as a launcher and front end. I have successfully installed and played a few games, however I am having an issue with one particular game, specifically NHL 98. Upon launching the game I get the following error:

"Developed For Use Only With NTSC Genesis Systems"

I understand that certain games have regional restrictions, however the ROM I am using is a USA ROM, and the settings in TKDgen 'system' tab are set to 'USA ntsc'. I have tried switching to PAL with no success.

Could someone explain to me how I might go about launching this wrong considering this error?

Thanks