problem of exponential function with s- function

I use s-function

my system is:

sys = [ u- 2*x(1) + x(2) - exp(40*x(1)) - exp(40*x(1) - 40*x(2)) + 2;

 x(1) - 2*x(2) + x(3) + exp(40*x(1) - 40*x(2)) - exp(40*x(2) - 40*x(3));

 x(2) - 2*x(3) + x(4) + exp(40*x(2) - 40*x(3)) - exp(40*x(3) - 40*x(4));

                         x(4) - x(3) - exp(40*x(3) - 40*x(4)) + 1];

when I press run, an error message appear (State derivatives returned by S-function 'tasksf' in 'Task1/S-Function' during flag=1 call must be a real vector of length 4). when I delete exp() from all equation it runs fine, what is the problem?