Overflow / math range error for log or exp

Line of code in question:

summing +=  yval * np.log(             sigmoid(np.dot(w.transpose(),xi.transpose()))) 
        +(1-yval)* np.log(max(0.001, 1-sigmoid(np.dot(w.transpose(),xi.transpose()))))

Error:

File "classify.py", line 67, in sigmoid
return 1/(1+ math.exp(-gamma))
OverflowError: math range error

The sigmoid function is just 1/(1+ math.exp(-gamma)).

I’m getting a math range error. Does anyone see why?