All publishers give this warning when a subscriber subscribes

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All publishers give this warning when a subscriber subscribes to them using wait_for_message:

[WARN] [1603436136.720705]: Inbound TCP/IP connection failed: connection from sender terminated before handshake header received. 0 bytes were received. Please check sender for additional details.

The publisher:

#!/usr/bin/python3
import rospy, time, math
import RPi.GPIO as GPIO
from geometry_msgs.msg import Twist
import sys, subprocess
"""subprocess.Popen([sys.executable, 'sudo mjpg_streamer -i "/usr/local/lib/input_uvc.so" -o "/usr/local/lib/output_http.so -w /usr/local/www"'])"""


GPIO.setmode(GPIO.BCM)

GPIO.setup(27, GPIO.OUT)
lf = GPIO.PWM(27, 50)
GPIO.setup(23, GPIO.OUT)
rf = GPIO.PWM(23, 50)
lf.start(0)
rf.start(0)

GPIO.setup(17, GPIO.OUT)
lb = GPIO.PWM(17, 50)
GPIO.setup(22, GPIO.OUT)
rb = GPIO.PWM(22, 50)
lb.start(0)
rb.start(0)

wheel_pwm = rospy.Publisher("wheel_pwm", Twist, queue_size=1)

def receiveMessage(msg):
    l = min((msg.linear.x + (-msg.angular.z)) * 50, 99)
    r = min((msg.linear.x - (-msg.angular.z)) * 50, 99)
    wheelpwm = Twist()
    wheelpwm.linear.x = l; wheelpwm.linear.y = r; wheelpwm.linear.z = 0
    wheel_pwm.publish(wheelpwm)
    if l >= 0:
        lf.ChangeDutyCycle(l)
        lb.ChangeDutyCycle(0)
    else:
        lb.ChangeDutyCycle(l * -1)
        lf.ChangeDutyCycle(0)

    if r >= 0:
        rf.ChangeDutyCycle(r)
        rb.ChangeDutyCycle(0)
    else:
        rb.ChangeDutyCycle(r * -1)
        rf.ChangeDutyCycle(0)



rospy.init_node('fourwd')
rospy.Subscriber('key_vel', Twist, receiveMessage)
rospy.spin()
GPIO.cleanup()

This is the subscriber:

#!/usr/bin/python3
import RPi.GPIO as GPIO
from geometry_msgs.msg import Twist
import rospy
from std_msgs.msg import Int16
from threading import Thread, Event

GPIO.setmode(GPIO.BCM)

GPIO.setup(24, GPIO.IN)
GPIO.setup(25, GPIO.IN)



pub = rospy.Publisher("wheels", Twist, queue_size=1)


rospy.init_node('pub_encoders')

event = Event()


def Tick(lr_):
    while True:
        lpwm = lr_[4]
        rpwm = lr_[5]
        if not lr_[2] and GPIO.input(24):
            lr[0]+=lpwm
            lr_[2] = True
        elif not GPIO.input(24):
            lr_[2] = False
        if not lr_[3] and GPIO.input(25):
            lr[1]+=rpwm
            lr_[3] = True
        elif not GPIO.input(25):
            lr_[3] = False
        rospy.sleep(0.01)
def Publisher(lr_):
    while True:
        _t = Twist()
        _t.linear.x = lr[0]
        _t.linear.y = lr[1]
        pub.publish(_t)
        rospy.sleep(0.1)

lr = [0,0, False, False, 1, 1]


reader = Thread(target=Tick, args=(lr,))
reader.start()

publisher = Thread(target=Publisher, args=(lr,))
publisher.start()


while True:
    try:
        twst = rospy.wait_for_message("wheel_pwm", Twist)
        if twst.linear.x > 0:
            lr[4] = 1
        elif twst.linear.x < 0:
            lr[4] = -1
        if twst.linear.y > 0:
            lr[5] = 1
        elif twst.linear.y < 0:
            lr[5] = -1
    except rospy.exceptions.ROSException:
        pass