rosserial fails when using sensor_msgs::Imu

edit

I am attempting to send IMU messages to ROS from an Arduino and was making good progress until last night... I can send all sorts of primitive messages, and messages which contain other primitives, but for some reason the IMU message doesn't make it to rostopic echo /imu. Code attached, please advise:

$ rosrun rosserial_python serial_node.py _port:=/dev/ttyACM0
[INFO] [WallTime: 1381881252.718444] ROS Serial Python Node
[INFO] [WallTime: 1381881252.726693] Connecting to /dev/ttyACM0 at 57600 baud
[INFO] [WallTime: 1381881255.329399] Note: publish buffer size is 512 bytes
[INFO] [WallTime: 1381881255.329896] Setup publisher on imu [sensor_msgs/Imu]
[ERROR] [WallTime: 1381881272.931531] Mismatched protocol version in packet: lost sync or rosserial_python is from different ros release than the rosserial client
[INFO] [WallTime: 1381881272.931971] Protocol version of client is Rev 0 (rosserial 0.4 and earlier), expected Rev 1 (rosserial 0.5+)

...

genpy.message.DeserializationError: unpack requires a string argument of length 32

Which is thrown from this line of _Imu.py:

(_x.orientation.x, _x.orientation.y, _x.orientation.z, _x.orientation.w,) = _struct_4d.unpack(str[start:end])

Here's my sketch code:

#include <ros.h>
#include <sensor_msgs/Imu.h>

ros::NodeHandle  nh;
sensor_msgs::Imu imu_msg;
ros::Publisher pub("imu", &imu_msg);
char frame_id[] = "imu";

void setup(){  
  nh.initNode();
  nh.advertise(pub);

  imu_msg.header.seq = 0;
  imu_msg.header.stamp = nh.now();
  imu_msg.header.frame_id = frame_id;


  imu_msg.orientation.x = 0;
  imu_msg.orientation.y = 0;
  imu_msg.orientation.z = 0;
  imu_msg.orientation.w = 0;

  imu_msg.orientation_covariance[0] = 0;
  imu_msg.orientation_covariance[1] = 0;
  imu_msg.orientation_covariance[2] = 0;

  imu_msg.orientation_covariance[3] = 0;
  imu_msg.orientation_covariance[4] = 0;
  imu_msg.orientation_covariance[5] = 0;

  imu_msg.orientation_covariance[6] = 0;
  imu_msg.orientation_covariance[7] = 0;
  imu_msg.orientation_covariance[8] = 0;


  imu_msg.angular_velocity.x = 0;
  imu_msg.angular_velocity.y = 0;
  imu_msg.angular_velocity.z = 0;

  imu_msg.angular_velocity_covariance[0] = 0;
  imu_msg.angular_velocity_covariance[1] = 0;
  imu_msg.angular_velocity_covariance[2] = 0;

  imu_msg.angular_velocity_covariance[3] = 0;
  imu_msg.angular_velocity_covariance[4] = 0;
  imu_msg.angular_velocity_covariance[5] = 0;

  imu_msg.angular_velocity_covariance[6] = 0;
  imu_msg.angular_velocity_covariance[7] = 0;
  imu_msg.angular_velocity_covariance[8] = 0;


  imu_msg.linear_acceleration.x = 0;
  imu_msg.linear_acceleration.y = 0;
  imu_msg.linear_acceleration.z = 0;

  imu_msg.linear_acceleration_covariance[0] = 0;
  imu_msg.linear_acceleration_covariance[1] = 0;
  imu_msg.linear_acceleration_covariance[2] = 0;

  imu_msg.linear_acceleration_covariance[3] = 0;
  imu_msg.linear_acceleration_covariance[4] = 0;
  imu_msg.linear_acceleration_covariance[5] = 0;

  imu_msg.linear_acceleration_covariance[6] = 0;
  imu_msg.linear_acceleration_covariance[7] = 0;
  imu_msg.linear_acceleration_covariance[8] = 0;
}

void loop(){

  imu_msg.header.seq++;
  imu_msg.header.stamp = nh.now();

  // Doesn't work:
  pub.publish(&imu_msg);

  // These work:
  // pub.publish(&imu_msg.header);
  // pub.publish(&imu_msg.orientation);
  // pub.publish(&imu_msg.angular_velocity);

  // This doesn't
  nh.spinOnce();
  delay(500);
}