buttons.py doesn't work: RuntimeError: Failed to add edge detection

[ctrlcctrlv]

I wrote a C version here: https://gist.github.com/ctrlcctrlv/5d75f1d8e7bb1dd32183074d06e3bbac

But you probably want to fix this, I'm not sure how to:

fred@kinomi:~/Workspace/inky/examples/7color $ ./buttons.py
buttons.py - Detect which button has been pressed

This example should demonstrate how to:
 1. set up RPi.GPIO to read buttons,
 2. determine which button has been pressed

Press Ctrl+C to exit!

Attaching falling edge watcher to pin 5 with bouncetime 250
Traceback (most recent call last):
  File "/home/fred/Workspace/inky/examples/7color/./buttons.py", line 42, in <module>
    GPIO.add_event_detect(pin, GPIO.FALLING, handle_button, bouncetime=250)
RuntimeError: Failed to add edge detection