Odometry causing a segmentation error

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Good afternoon, I have to control a differential drive robot. I had not to use the differential controller plugin, so i just implemented two velocity controllers and a joint state. I now have to compute odometry for my robot and I want to use my joint state as a velocity sensor for each wheel. So i would subscribe /joint_state, take the two velocities and the compute odometry from them and publish all on a /odom topic. The building works, but once i run this script in my ros network I have just some loops and then a core dump error when the callback is called. Could you help me, please? Here is my header file:

#include <ros/ros.h>
#include <tf/transform_broadcaster.h>
#include <nav_msgs/Odometry.h>
#include <sensor_msgs/JointState.h>
#include <math.h>
#include "boost/thread.hpp"
#include "tf/tf.h"
using namespace std;

class ODOM{
    public:
        ODOM();
        void run();
        void encoders_sub_callback (sensor_msgs::JointState msg);
        void compute_odom ();

    private:
    ros::NodeHandle _nh;
         float _wl;
         float _wr;
        long double _x;
        long double _y;
        long double _th;
        bool _first_odom;

        ros::Subscriber _encoders_sub;
        ros::Publisher _odom_pub;

};

and here my cpp file:

#include "odom.h"

ODOM::ODOM(){
    _encoders_sub = _nh.subscribe("joint_states", 1, &ODOM::encoders_sub_callback, this);
    _odom_pub = _nh.advertise<nav_msgs::Odometry>("/odom",1);
    _wl=0.0;
    _wr=0.0;
    _x=0.0;
    _y=0.0;
    _th=0.0;
    _first_odom=false;
}

//here it happens the segmentation fault

void ODOM::encoders_sub_callback(sensor_msgs::JointState msg){
    cout<<"callback"<<endl;
    _wl= msg.velocity[0];
    cout<<_wl<<endl;
    _wr=msg.velocity[1];
    cout<<_wr<<endl;
}

//the computing odom function//...

... and then...

void ODOM::run(){

    boost::thread odom_t( &ODOM::compute_odom, this);

    ros::spin();
}

int main(int argc, char** argv){
 ros::init(argc,argv, "odom_node");

    ODOM odom;
    odom.run();


    return 0;
}