using a Prewitt filter to detect edges in a image

Hi i am trying to do something basic just to acquint myself with image processing. I'm a newbie at this. I am just trying to use a Prewit filter to find the edges; This is the code that I have used:

ic = imread('ic.tif');

b = mean(ic,3);

px = [-1 0 1; -1 0 1; -1 0 1];

ic = filter2(px,ic);

figure, imshow(icx/255)

i get the statement:

Undefined function 'conv2' for input arguments of type 'double' and attributes 'full 3d real'.

before converting the image to a double it was saying that the filter function is 2d and the image is 3d. I am nt sure what to do . Please help.

[Image Analyst]

What does this say:

whos ic

and what is icx - you never defined it.