Cannot create 'F' since [opti12_x_1] are free.

[Prakitr Srisuma]

In MATLAB, I am trying to pass the decision variable into the ODE solver (see code below) to calculate the objective function. However, this is the error message I got:

Error using casadi.integrator
.../casadi/core/integrator.cpp:58: Cannot create 'F' since [opti12_x_1] are free.

Does anyone know how  to resolve this?

Tb_opt = opti.variable(); % decision variable

x = MX.sym('x');

dae = struct('x',x,'ode',ODEfunc(x,Tb_opt));

F = integrator('F', 'cvodes', dae);

r = F('x0',0);

objfunc = r.xf;