parallel computing parfor error?

Hi guys, I want to use parallel computing. However, I could get rid of the common error although I did some reading about variables constraints. Could someone tell me the reason? Thanks.

parfor nb=1:s b(:,nb)=rand(c,1); exist=find(b(:,nb)); num=length(exist(:)); for m=1:L r=rand(num); for i=1:num bnow(exist(i),:,m,nb)=w(exist(i),:,m,nb).^2; Error: The variable b in a parfor cannot be classified. If I use parfor for loop m, then I will also get a similar error. Error: The variable bnow in a parfor cannot be classified.