how to find coordinate of corner of label image

Please help this problem :

i have problem , i already process my image : <http://tinypic.com/view.php?pic=2czer0o&s=6>

and then this my image after process label : <http://tinypic.com/view.php?pic=fbgcbm&s=6>

but have message error : ? ?? Function 'find' is not defined for values of class 'cell'. Error in ==> mathcorner at 67 upperLeftCornerX = find(B(1,:), 1, 'first'); __________________________

error found when i tried to find corner coordintae of label image .. this my code :

    %clear all;
    %close all;

    rgbImage = imread('IM.jpg');
    % Get the dimensions of the image.  numberOfColorBands should be = 3.
    [rows columns numberOfColorBands] = size(rgbImage);

    % Extract the individual red, green, and blue color channels.
    % redChannel = rgbImage(:, :, 1);
    % greenChannel = rgbImage(:, :, 2);
    blueChannel = rgbImage(:, :, 3);
    imshow(blueChannel, []);
    % Threshold blue channel
    binaryImage = blueChannel < 198;
    % Get rid of small blobs.
    binaryImage = bwareaopen(binaryImage, 10000);
    % Fill in any holes in the tile.
    binaryImage = imfill(binaryImage, 'holes');

    % Display it.
    imshow(binaryImage, []);

    % Display it.
    imshow(binaryImage, []);
    SE = strel ('disk', 5);
    i1 = imerode(binaryImage, SE);
    figure, imshow(i1, []);

    BWc1 = imclearborder(i1,1);
    figure, imshow(BWc1, []);

    SE = strel('disk', 2, 0)
    i1 = imclose (BWc1, SE);
    figure, imshow(i1, []);

    [x, y, rgb] =ind2sub ([size(i1,1) size(i1, 2) size(i1,3)], find (i1~= 255));
    A = i1 (min(x): max(x)-1, min(y):max(y)-1,:);

    M = im2double(A);
    e1 = edge (M, 'canny');
    figure, imshow(e1, []);

    wx = [-1 -2 -1; 0 0 0; 1 2 1];
    wy = [-1 0 1; -2 0 2; -1 0 1];
    wx1= [-2 -1 0; -1 0 1; 0 1 2];
    wy1= [0 1 2; -1 0 1; -2 -1 0];

    gx= imfilter(double(e1),wx, 'replicate');
    gy= imfilter(double(e1),wy, 'replicate');
    gx1= imfilter(double(e1),wx1, 'replicate');
    gy1= imfilter(double(e1),wy1, 'replicate');

    g = gx + gy + gx1+ gy1;
    figure, imshow (g, []);

    [B,L] = bwboundaries(g,'holes');

    % Display the label matrix and draw each boundary
    imshow(label2rgb(L, @jet, [.1 .1 .1]))
    hold on
    for k = 1:length(B)
      boundary = B{k};
      plot(boundary(:,2), boundary(:,1), 'w', 'LineWidth', 1)
    end

%find coorndinate ,,
  upperLeftCornerX = find(B(1,:), 1, 'first');
  upperRightCornerX = find(B(1,:), 1, 'last');
  lowerLeftCornerX = find(B(end,:), 1, 'first');
  lowerRightCornerX = find(B(end,:), 1, 'last');

    upperLeftCornerX 
    upperRightCornerX 
    lowerLeftCornerX 
    lowerRightCornerX