Bizarre joke

During last Christmas a family member told me the following joke. They said it was one that they had heard at Halloween, and could not work out why on earth it was a funny joke. Here it is:

"What do ghosts eat for breakfast? Ghost cereal!"

No one could work out how it became a joke, or why it was even considered funny (if anything, we laughed at how not funny it was). "Ghost cereal" does not appear to be a play on words like "boo-berry" or "ghoulash"; in addition, a key word in the joke question (ghost) is also being used in the punchline, which often ruins a joke.

Just now I Googled "ghost cereal", and I found this in Google Images. However, all I saw was what looked like regular brands that merely adopt a "ghost" theme for Halloween, but nothing that gave any meaningful insight into the origins of the joke.

Any ideas?