Improper Index Matrix - what is going on?

I'm getting an improper index matrix error for the following script:

    % Ysum= zeros(V(1).dim)';
for t = 1:length(V)
    Ysum = Ysum + spm_read_vols(V(t));
end

With 335 functional MRI images loaded in and onset times locked in. Is there anything anyone can think of to fix this??

[Matt J]

We need to see the error messages. Please copy/paste. Most likely, V contains non-integer values or something like that.