"Conversion to Cell from Double Is Not Possible"

I am trying to find features of EEG signals using TQWT. For finding the features of many columns I tried following code, yet I am getting the error:

Conversion to cell from double is not possible.

The code is:

for k = 1:9
  filename = sprintf('F00%d.txt',k);
  a(:,k) = load(filename);
  temp = a(:,k);
  x = temp(2:length(a(:,k)));
  w = tqwt(x,1,3,3);
  [a1,a2,a3,a4] = deal(w{:});
  a = {a1, a2, a3, a4};
  for j = 1:4
    H(k,j) = KraskovEntropyV2((a{j})', 2, 'euclidean');   
    j=j+1;
  end       
end

What can I do about this error?