Paint bucket and wand are selecting empty pixels for some reason

I’m learning Aesprite and finding it pretty straight forward for the most part. However, I’ve created one document and somehow got into a state where if I use the paint bucket or wand tool on a color region that touches the empty parts of the background (ie, the checkboard pattern), it flood fills/selects the empty area as well as the shape. This doesn’t happen if I start a new document, though.

Any idea what I might have done to enter this state?