why is `sudo pkill -HUP -f "nginx: master process"` returning code 129 and no output

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http://unix.stackexchange.com – I'm trying to trigger a reload of the nginx master process by using pkill and sudo. The server reloads fine, but I was just curious if anyone knows why the command `sudo pkill -HUP -f "nginx: master process" returns code 129. # Running as root $ pkill -HUP -f "nginx: master process" $ echo $? # Output is as expected: 0 # Running as a user (sudo is set to not prompt for a password) % sudo pkill -HUP -f "nginx: master process % echo $? # Output is weird (considering sudo should be passing along the return # code of the command it is executing): 129 Running on Ubuntu 12.04. (HowTos)