Photoshop :: Copy/past Tool

I try to use the lasso tool to select an irregular chunk of an image and copy this chunk to another part of an image.

When I do this, it says "Could not complete the copy command because the selected area is empty." But there is clearly something in the selected area, and even if there wasn't, it should still copy white space.

This worked once, but when I try to do it now, I have the above problem. I don't know what I did different the time that it worked.

I'd use Microsoft Paint to do this, but it only lets me copy and paste square
chunks and I need irregular.