concentration mode

I just tried my spec 20D+.  I just inhereted this classroom and found the machine.  seems to work and I have manual.  I cannot get the concentration to work.  I put in a blank that was  I molar and set the calibaration.  when I put in the "unknown" I got a readingof -54.2.  What does that mean?  Does it mean .5 Molar?  thanks for your help