Strange behavior of JointVelocityController in rrbot tutorial

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Hi, the original rrbot module works fine as indicated in the tutorial. But the problem comes when I just change the joint controller of one joint (here, the joint1) from the position controller to the velocity controller (in the rrbot_control.yaml file and also in the controller launch file) and do not touch any other things, and then loads the controller:

joint1velocitycontroller: type: effort_controllers/JointVelocityController joint: joint1 pid: {p: 100.0, i: 0.01, d: 10.0}

The program either crashes:

Physics dynamic reconfigure ready. gzclient: /build/buildd/ogre-1.7.4/OgreMain/include/OgreAxisAlignedBox.h:252: void Ogre::AxisAlignedBox::setExtents(const Ogre::Vector3&, const Ogre::Vector3&): Assertion `(min.x <= max.x && min.y <= max.y && min.z <= max.z) && "The minimum corner of the box must be less than or equal to maximum corner"' failed.

or the whole robot looks strange as all the links are combined together (see capture here: https://docs.google.com/file/d/0B49go8f2Nr2gTTJzdl80ckxoOFk/edit?usp=sharing), and it does not rotate anymore. Thanks for any help!