idss is a variable with only one element. The expression idss(1-exp((x))) is an attempt to access one or more elements from that variable. The only valid index into idss is 1 (since it has only one element) and for 1-exp(x) to result in a value of 1, x must be -Inf (or something that causes exp(x) to underflow to 0.)
If you meant to multiply the variable idss and the value of the expression 1-exp(x), you need to specify the multiplication explicitly by putting .* between those two terms.