Covariance of Images data set

 I have the code below:

   Np=40;
   TI = 10;           % number of images per person
   for NTr = 1 : 9;
   c=1;
      for i=1:NP
          for j=1:NTr
              im = ['face' int2str(rndMtrx(itr,(i-1)*TI+j)) '.bmp'];
              [X,MAP]=imread(im);
              x=ind2gray(X,MAP);
              Train(:,c) = x(:);%double(x(:))/norm(double(x),2);   % norm(x,2) for normalization.      
              c=c+1;
          end
      end
then i should calculate the covariance of the iamges :

          covar= cov(Train);

Iam getting the error below:

*meaError using bsxfun
Mixed integer class inputs are not supported.

Error in cov (line 154)
xc = bsxfun(@minus,x,sum(x,1)/m);  % Removen*

any help please?