Help: Edge detection technique of image

I have used the following code

close all;

a = imread('3.bmp');

imshow(a)

figure

imedge = edge(img, 'sobel',0.05);

imshow(imedge, 'ColorMap', [1 1 1; 1 0 0]);

figure

title('sobel');

img = imread('3.bmp');

imedge1 = edge(img, 'prewitt',0.05);

imshow(imedge1, 'ColorMap', [1 1 1; 1 0 0]);

figure

title('prewitt');

img = imread('3.bmp');

imedge2 = edge(img, 'canny',0.05);

imshow(imedge2, 'ColorMap', [1 1 1; 1 0 0]);

figure

title('canny')

resulting in the following error:

Attempt to execute SCRIPT image as a function:

C:\Users\Administrator\Documents\MATLAB\image.m

Error in images.internal.basicImageDisplay (line 24)

hh = image(cdata, ...

Error in imshow (line 321)

hh = images.internal.basicImageDisplay(fig_handle,ax_handle,...

Error in m6 (line 3)

imshow(a)

Please help me to sort out this error. Any help is much appreciated.