When pr2.launch file is launched, gazebo crashes [closed]

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Hello,

In the following simple scenario:

1) Launched empty_world.launch file satisfactorily (with the command: roslaunch gazebo_worlds empty_world.launch)

2) When launched pr2.launch file (with the command: roslaunch pr2_gazebo pr2.launch)

I get the following message from the terminal from which I launched empty_world.launch:

sh: 1: prefix: not found

terminate called after throwing an instance of 'pluginlib::LibraryLoadException' what(): rospack could not find the pr2_controller_interface package containing pr2_controller_interface::Controller Aborted (core dumped)

And the PR2 is not shown on the gazebo window.

What does "sh: 1: prefix: not found" mean?
I'd appreciate if anyone could help me solve this problem.

Basic info about my set-up:
- ubuntu 12.04 (precise) 64-bit
- GeForce GT 520
- NVIDIA Driver Version: 302.17
- ROS distribution: fuerte

Thanks!