Fixing error message that results instead of a line plot

Question

I am trying to run the function to perform a plot but instead, I am getting the error:

Message[With::lvset, {dt = 0.01, tfinal = 100., \[Gamma] = 1, k = 1, 
  Subscript[\[Sigma], 1] = 1, Subscript[\[Sigma], 2] = 2}, 
 Subscript[\[Sigma], 1] = 1, Subscript[\[Sigma], 1]]

The function, which is a for loop that stores the values in a list and plots them is :

With[{dt = 0.01, tfinal = 100.0, \[Gamma] = 1, k = 1, 
  Subscript[\[Sigma], 1] = 1, Subscript[\[Sigma], 2] = 2},
 x = {0, 0};
 a = 0.0;
 alist = {};
 dtsqrt = Sqrt[dt];
 M = ( {
    {1 + 1, -1},
    {-1, 1 + 1}
   } );
 \[Sigma] = ( {
    {1, 0},
    {0, 2}
   } );
 Do[
  dx = -M.x*dt + 
    dtsqrt*\[Sigma]. RandomVariate[NormalDistribution[], 2];
  a += x[[1]] dx[[2]] - x[[2]] dx[[1]] ;
  x += dx;
  AppendTo[alist, {t, a}], {t, dt, tfinal, dt}];
 p1 = ListPlot[alist, Joined -> True];
 Show[p1]
 ]

Could you tell me where the error emerges from or perhaps how to stop it from occurring?

Answer 1

Don't use Subscript to name your variables. Name them $a1$ and $a2$ for example. This would solve the problem.