allow_free option not working

[dgnsilva]

Hello everyone.

I was trying to write to code a simple Control Lyapunov Function based controller in MATLAB, and came across an error message saying I needed to allow "free_variables".

Initialization failed since variables [delta] are free. These
symbols occur in the output expressions but you forgot to declare
these as inputs. Set option 'allow_free' to allow free variables.

Error in untitled5 (line 44)
solver = qpsol('solver', 'mosek', qp_prob, opts);

And when I tried to parse this option to qpsol, nothing seems to change and I still get the same error.

% ---- Decision variables ----
X = SX.sym('X', nx); %multiple state variables
U = SX.sym('U', nu); %multiple control variables
delta = SX.sym('delta');

% ---- CLF variables ----
rho = 1; % Relative importance of u cost and x cost
gamma = 1; % How fast to converge
w = 1; % How agressive the controller is

% ---- Compute solution symbolically, objective and constraints ----
obj = U^2 + rho*delta^2;
g = (2*(X(1)-ref)*X(2) + 0.5*w*X(2)^2) + (0.5*w*(X(1)-ref) + 2*w*X(2))*U + delta + gamma * ((X(1)-ref)^2 + 0.5*w*(X(1)-ref)*X(2) + w*X(2)^2);

% ---- Set solver options ----
OPT_variables = [X', U];

opts = struct;
opts.allow_free = true;

qp_prob = struct('f', obj, 'x', OPT_variables, 'g', g);
solver = qpsol('solver', 'mosek', qp_prob, opts);

Am I doing something wrong? Do I need to declare this option somewhere else in a different way?
Any help is greatly appreciated.