Why can't I contract my selection from the image's edge?

When I make a selection that includes pixels on the edge of the image, then click "select" -> "modify" -> "contract", the selection is only contracted on the edges that are NOT on the images edge. The selection remains up against the edge of the photo.

This is in Photoshop CC, and I used to be able to do this, so I'm not sure if I accidentally changed a setting, or if an update caused this, but I'd like to fix it please!