cannot read all the frame in a video

"""""

obj=VideoReader('333.wmv'); %read video

class(obj);

a=read(obj);

frames=get(obj,'numberOfFrames');

   for k = 1 : frames

       I(k).cdata = a(:,:,:,k);

       I(k).colormap = [];

   end
for j = 1: obj.NumberOfFrame-1     
    b=read(obj,j);        
    c=read(obj,j+1);
end

"""

this is my coding to read the video and it is success to read the video but my PROBLEM is i when i used a for loop to read the number of frame it pop out some error

"""""

Error using VideoReader/read (line 86)

The frame index requested is beyond the end of the file.

Error in tryerror (line 15)

    b=read(obj,j);

""""""""

anyone can help me to solved this problem My aim is try to extract out each of the frame and by running a loop i can calculate the different between different frame....