Error using parfor: cannot use parfor because of the way Theta/nk is used

Hello everyone, I am using a very old version of Xeon PC to run a gaussian mixture model using Dirichlet process and Gibb's sampling on a Big Data. I hoped I could speed it up using parfor commands but was unable to do it becuase of the error!!

 n = size(X,2);
Theta = {};
nk = [];
label = zeros(1,n);
llh = 0;
for i = randperm(n)
    x = X(:,i);
    Pk = log(nk)+cellfun(@(t) t.logPredPdf(x), Theta);
    P0 = log(alpha)+theta.logPredPdf(x);
    p = [Pk,P0];
    llh = llh+sum(p-log(n));
    k = discreteRnd(exp(p-logsumexp(p)));
    if k == numel(Theta)+1
        Theta{k} = theta.clone().addSample(x);
        nk = [nk,1];
    else
        Theta{k} = Theta{k}.addSample(x);
        nk(k) = nk(k)+1;
    end
    label(i) = k;
end
w = nk/n;

Please note that the first function I used above provided faster results so, I am not worried about it very much.

Thanks and Regards in advance.