Comment 2 for bug 1506715

I get that, too (0.91, LM 17.2, 64bit).
Status line says 'area is not bounded, cannot fill'. Zoom out just a tiny bit or move the object a little more onto the canvas and it will work.

The manual says
"Since the Paint Bucket Tool uses pixels to calculate the area to be enclosed, Inkscape will clip the region off-screen to prevent the number of pixels that go into the filling algorithm from becoming too large. If this happens you can either zoom out to do the fill or you can do the fill in pieces. "
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Bucket-Union.html (also see images there)

So from the manual, I conclude that it is expected that the bucket tool fills until the borders of the visible canvas (probably only *if* the area is bounded? But how will it know the difference?) or until a color change.
But if the area-to-fill is outside the visible area by a certain amount, it just doesn't fill anything.
'Doing the fill in pieces' doesn't appear to be possible.