How to get background subtraction in video?

I'm trying to get background subtraction to work on a video. I have an mp4 file and an image but when running the code it gives this error -

*_Index in position 2 is invalid. Array indices must be positive integers or logical values.

Error in test1 (line 12) bg = mean( J(4, vid{:}), 4 );_*

The code is:

apple=imread('apple2.jpg');

%// read the video:
reader = VideoReader(apple4.mp4');
vid = {};
while hasFrame(reader)
    vid{end+1} = im2single(readFrame(reader));
end
%// simple background estimation 
bg = mean( apple(4, vid{:}), 4 );
%// estimate foreground as deviation from estimated background:
fIdx = 43; %// do it for frame 43
fg = sum( abs( vid{fIdx} - bg ), 3 ) > 0.25;

figure;
subplot(131); imshow( bg ); 
subplot(132); imshow( vid{fIdx} );
subplot(133); imshow( fg );

How can I get this code to work?

Thanks