Could not change --background after --crop

[tonyng3101]

After crop, I want to change the background color after crop of the gif but I got an error
gifsicle: warning: irrelevant background color
(The background will appear transparent because the first image contains transparency.)
gifsicle:cc0a3ff83fe53c73b12f2fdbd9ebe99b.gif: warning: background color not in colormap

Is have anyways to add a background to transparency after crop?