real time image processing (while loop)

hello im tring to write a real time motion detection matlab code but i have a weird problem with the while loop this is a part of the code

    im=getsnapshot(vid); 
    while(i<m)
    flag=0;
    pause(.1)
    im2=getsnapshot(vid);
    im=imcrop(im,[mesh(i,1) mesh(i,2) (mesh(i,3)-mesh(i,1)) (mesh(i,4)-mesh(i,2))]);
    im2=imcrop(im2,[mesh(i,1) mesh(i,2) (mesh(i,3)-mesh(i,1)) (mesh(i,4)-mesh(i,2))]);
     im=rgb2gray(im);
    im2=rgb2gray(im2);
fr_diff = abs(double(im) - double(im2));

but i get this error

Error using rgb2gray>parse_inputs (line 81) MAP must be a m x 3 array.

Error in rgb2gray (line 35) X = parse_inputs(varargin{:});

Error in testing (line 27) im=rgb2gray(im);

when the line im=getsnapshot(vid); is inside the loop it doesn't give an error!!!!

Answer 1

[Image Analyst]

What does

whos im
size(im)

say? It might be more robust to do this

[rows, columns, numberOfColorChannels] = size(im);
if numberOfColorChannels > 1
  im = rgb2gray(im); % Convert color to gray scale.
end

Answer 2

[naim]

it is an rgb image when i change the code to

     while(i<1)
     im=getsnapshot(vid);
.
.
.

 the code works with no errors O_o

Answer 3

[naim]

ok i got it now i save the gray croped image in the same matrex (im) -.- XD ok thanks man :D