[Gimp-user] Bucket Fill Tool won't filled selected

[billlee <forums gimpusers com>]

GIMP 2.8.6 on Window 8.

I used Canvas Size to add a horizontal space at the bottom of a jpeg photo. (I
want to use it for a text caption for the photo.) The new space is filled with a
checkerboard of small light & dark grey squares.

I used the Rectangle Select Tool to select the new space.

I clicked on the Bucket Fill Tool and moved the mouse over the new space. The
mouse pointer includes a circle with a diagonal stroke through it, indicating I
cannot fill the selected area.

What am I doing wrong or missing?? 
(Probably something simple--I'm a GIMP newbie....)

Many thanks!!
Bill

-- 
billlee (via www.gimpusers.com/forums)