Unable to communicate with Arduino using ROSSerial client

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I'm trying to get the basic ROSSerial Blink example to work. I managed to install, compile and upload everything just fine, but when I go to run the final command:

rosrun rosserial_python serial_node.py /dev/ttyACM0

it appears to time-out with the error:

[ERROR] [WallTime: 1450064686.648424] Unable to sync with device; possible link problem or link software version mismatch such as hydro rosserial_python with groovy Arduino

My target platform is an Arduino Leonardo, and I've uploaded a non-ROS blink example that outputs to the Serial line, so I know it works.

What am I doing wrong? Does serial_node.py need some "special" parameter to get it to work with a Leonardo?

I'm using Indigo on both the Arduino and host PC.