Failure in initial objective function evaluation. FMINCON cannot continue HELP

Hi, I have to solve the problem in the image but my fmincon doesn't work. Someone could help me? My code attached. Thanks in advance!

n_asset = size(Assets,2);

Aeq = ones(1,n_asset);
beq = 1;
x0 = mean(Returns);
lb = zeros(1,n_asset);
ub = ones(1,n_asset);

fun = @(x) -(x')*(sqrt(variance))/sqrt((x')*Covariance*(x))

w = -fmincon(fun,x0,[],[],Aeq,beq,lb,ub)

[Sergey Kasyanov]

What are sizes of variance and covariance?

[Steve Grikschat]

fmincon calls the objective with x0 before starting and that's where this error occurs.

Try evaluating fun with x0 before calling fmincon. That will give you the specific error.