Octave Error: Subscript indices must be integers or boolean

I was trying to solve a system of non-linear equations and check the stability of its equilibrium points. Initially, I declared two equations

adot = -a + 2*a^3 + b
bdot = -a -b

By equating both the equations with 0, I get the equilibrium points. Now, I am trying to get the Jacobian of the [adot;bdot] matrix using the jacobian([adot;bdot],[a,b]) method of symbolic package in Octave, which should just return a matrix whose items are the partial derivatives of "adot" and "bdot" w.r.t. "a" and "b", but it gives the following error

error: subscript indices must be integers or boolean

Can anyone tell me where I'm going wrong with this?

Edit: I'm adding the complete code down below:

pkg load symbolic

syms x y

xdot = -x + 2*x^3 + y;
ydot = -x - y;

[xeq,yeq] = solve(xdot==0,ydot==0);

xeq = double(xeq);
yeq = double(yeq);

jacobian_matrix = jacobian([xdot;ydot]);

At this point I'm getting the above mentioned error. The values in the [xeq,yeq] matrix are the equilibrium points of the system, which are to be used later on.