Error with ismember in a loop for

Hi, I have the following code with the error: "Input A of class cell and input B of class cell must be cell arrays of strings, unless one is a string".

for ii=1:size(couple,1);
    looking_up=couple(ii,:);
[tf, coldix]=ismember(looking_up,ff(ii,:));
prices=ff(:,coldix(tf));
prices=prices(2:end,:);
prices=cell2mat(prices);

x(ii)=prices(:,2);
y(ii)=prices(:,1);
end

where prices is a nxn matrix

stock_1 stock_2 ..... stock_n

45.5.....88.9......555.9

and so on

couple is a nx2 matrix:

stock_1 stock_2
stock_5 stock_8

and so on

[Walter Roberson]

But what is ff?