Photoshop :: Create A Pattern In CS5?

I can't create a Pattern of a selection area. When I try to create the Pattern at Edit->Define Pattern, this option isn't available. I will tell you the steps I'm taking:

->Create a new doc and use Paint Bucket Tool to paint it with Black (this is the background).
->Then I create a new layer and with the rectangular marquee tool I do a square.
->Keeping the selection I press cmd+backspace and the square becomes white.
->Keeping the selection I do a Stroke with 2px, color black and option Inside.
->Then I go to Edit to define a pattern of that square, however the Define Pattern option is gray, so I can't use it.