Bogofilter with Claws Mail, seems not to act on spam

[Adrian]

Ok, this definitely isn't off-topic!

bogofilter -t -v <knownspam.txt 
U 0.520000

bogofilter -t -v <knownspam.txt 
U 0.520000

bogofilter -t -v <gibberish.txt 
U 0.520000

bogofilter -t -v <empty.txt 
U 0.520000

What gives?  I guess I'm not testing the files I'm passing.