Code not compiling on OSX : vector 18 error??

I am trying to compile the code:
http://blog.wingedvictorydesign.com/2009/03/20/receive-dmx-512-with-an-arduino/3/

The author says he verified that it worked on IDE16 and I followed his instructions of replacing HardwareSerial.cpp with his own version.

When I try to compile I get the following messages:

/var/folders/-C/-CrtojkWGECwwQB8ALrEbE+++TI/-Tmp-/build9075536894274775616.tmp/core.a(HardwareSerial.cpp.o): In function `__vector_18':

/Applications/arduino-0016/hardware/cores/arduino/HardwareSerial.cpp:95: multiple definition of `__vector_18'

o:/var/folders/-C/-CrtojkWGECwwQB8ALrEbE+++TI/-Tmp-/build9075536894274775616.tmp/Temporary_2917_844.cpp:174: first defined here
o to 132 in /var/folders/-C/-CrtojkWGECwwQB8ALrEbE+++TI/-Tmp-/build9075536894274775616.tmp/core.a(HardwareSerial.cpp.o)
Couldn't determine program size: hardware/tools/avr/bin/avr-size: '/var/folders/-C/-CrtojkWGECwwQB8ALrEbE+++TI/-Tmp-/build9075536894274775616.tmp/receiver_rev13.hex': No such file

Any idea what might be causing this???

UPDATE: this same code compiles fine on a PC, it is only on OSX that I get these errors.