Writing Stone-Geary preferences into a utility function

Fellow GAMS users,

I am trying to introduce a subsistence parameter (Stone-Geary preferences) into my model’s utility function. Unfortunately when I introduce this parameter the model will no longer run (I’m simply told the solver was aborted).

I was hoping one of you might have some experience implementing Stone-Geary preferences in GAMS and could tell me where I’m going wrong.
The model is based on Ramsey’s growth model.

The utility function is as follows

utility(t)$(ord(t) lt card(t))…
util(t) =e= alpha*log(Cf(t)-Cfmin)+(alpha)*log(C(t)) ;

alpha: elasticity parameter =0.5
C(t)= Consumption of production good
Cf(t)= Consumption of food good
Cfmin = Subsistence parameter = 0.3