rostopic echo /chatter doesnt work

edit

I created a message file and tried to publish it by the publisher file and also i created a subscriber in another file to get it.

the sum.msg is located in msg folder and looks like this: i

 int32 a
 int32 b

the publisher file is like this :

pub = rospy.Publisher('chatter', sum, queue_size=10)
msg = sum()
msg.a= 1
msg.b= 2
rospy.loginfo(msg)
pub.publish(msg)
time.sleep(5)
rospy.spin()

and the subscriber is like this:

def callback(data):
        print (" i'm in callback!")
        global a
        global b      
        a=data.a
        b=data.b
        ros_info("a: %d, b: %d",a,b)

.
.

.
def main():
rospy.Subscriber("chatter", sum, callback)
    rospy.spin()

also i added the sum.msg file to my CMakelist.txt file.

but the problem is that when i run rostopic echo /chatter

i get this error:

[WARN] [WallTime: 1532522302.936274] Inbound TCP/IP connection failed: connection from sender terminated before handshake header received. 0 bytes were received. Please check sender for additional details.

So can anyone help me to fix it ..

I'm using ubuntu 14.04 and ros indigo and used python