Get error when trying to publish float64 arm message

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Get an error(see below) when trying to publish a float64 arm command.

Using Ubuntu 12.04.1 Ros Hydro

source file:

#include "ros/ros.h"
#include <std_msgs/Float64.h>
#include "pub/ArmPan.h"

pub::ArmPan panmsg;

int main(int argc, char **argv)
{
    ros::init(argc, argv, "pub");
    ros::NodeHandle n;
    ros::Publisher ArmPan_pub = n.advertise<std_msgs::Float64>("ArmPan/command", 1000);
    ros::Rate loop_rate(10);
    int count = 0;

    while(ros::ok())
    {
        panmsg.ArmPan = 1.0;
        ArmPan_pub.publish(panmsg);
        ros::spinOnce();
        loop_rate.sleep();
        ++count;
    }
    return 0;
}

the file ArmPan.msg has one line: float64 ArmPan

rosrun fails with this err:

[FATAL] Assertion failed
file = /opt/ros/hydro/include/ros/publisher.h
line 115
cond = impl_->md5sum == "*" || std::string(mt::md5sum<M>(message))
             == "*" || impl_->md5sum == mt::md5sum<M>(message)
Trying to publish message of type [pub/ArmPan] on a publisher with type [std_msgs/Float64]

if the message

rostopic pub -1 ArmPan/command std_msgs/Float64 1

is entered, it works.

This code worked in earlier versions of ROS but now fails with Hydro. Can you tell me why? Thanks.