Please help me to fix the CVX error. I am maximizing a provably concave function below.

n_dash, M, and Nf are integers. q_i is a row of size 1*Nf and the decision vector is a vector x of size Nf*1.

```
cvx_begin
variables x(Nf)
maximize ( q_i*x + n_dash*q_i*( x.*(1-x).*exp(-n_dash*x) ) )
subject to
sum(x) == M
x>= zeros(Nf,1)
x<= ones(Nf,1)
cvx_end
```