"Variable in a parfor cannot be classified" error.

I am trying to convert my code over to run with parfor, since as it is it takes a long time to run on its own. However I keep getting this error. I have search around on the website and have read people with similar problems, but none of those answers seem to fix my problem. This is my code:

r = 5;

Mu = 12.57e-9;

Nu = 12e6;

I = 1.8;

const = pi*Nu*Mu*r*I;

a = 55;

b = 69;

c = 206;

[m,n,p] = size(Lesion_Visible);

A = zeros(m,n,p);

parpool(2)

syms k

parfor J = 1:m

      for  I = 1:n

          for K = 1:p

               if Lesion_Visible(J,I,K) ~= 0
                  Theta = atand((J-b)/(I-a));
                  Rho = abs((I-a)/cosd(Theta))*0.05;
                  Z = abs(c-K)*0.05;
                  E = vpa(const*int(abs(besselj(0,Rho*k)*exp(-Z*k)*besselj(0,r*k)),0,20),5);
                  A (J,I,K) = E;
              end
          end
      end
  end

I'm trying to calculate the electric field in specific position on an array and matlab give me the error "The variable A in a parfor cannot be classified". Thanks.

Answer 1

[Walter Roberson]

parfor J = 1:m

      A_slice = zeros(n, p, 'sym');

      for  I = 1:n

          for K = 1:p

               if Lesion_Visible(J,I,K) ~= 0
                  Theta = atand((J-b)/(I-a));
                  Rho = abs((I-a)/cosd(Theta))*0.05;
                  Z = abs(c-K)*0.05;
                  E = vpa(const*int(abs(besselj(0,Rho*k)*exp(-Z*k)*besselj(0,r*k)),0,20),5);
                  A_slice(I,K) = E;
              end
          end
      end

      A(J,:,:) = double(A_slice);
  end