Other Dependents Tax Credit

I have a son who's older than 17, who did not work all of last year - he lived the entire year in a house I own, and I supplied 100% of his financial support - the Turbotax description of situations that allow the other dependents tax credit seems to imply that I am eligible for this credit, but when I try to get it, the S/W is telling me I'm not eligible because I don't  have child dependents - this seems to make no sense... Can someone explain if I'm eligible for the credit, and if not, why not?