Raspberry Pi, Python, XBee to Arduino Xbee.write() error

I have a problem with my Raspberry Pi talking to my Arduino through XBee modules.

I have Python script running a websocket, from where i wish to control the pins of my Arduino and Raspberry Pi through a website.

Whenever i call the commands for controlling the Raspberry Pi, everything is fint. But when i call the commands that should be send through Serial, the script breaks, and gives me this error:

ERROR:tornado.application:Uncaught exception in /ws
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/tornado/websocket.py", line 303, in wrapper
    return callback(*args, **kwargs)
  File "websocket.py", line 101, in on_message
    xbee.write("%s" % message)
  File "/usr/local/lib/python2.7/dist-packages/serial/serialposix.py", line 491, in write
    d = to_bytes(data)
  File "/usr/local/lib/python2.7/dist-packages/serial/serialutil.py", line 76, in to_bytes
    b.append(item)  # this one handles int and str for our emulation and ints for Python 3.x
TypeError: an integer or string of size 1 is required
Connection was closed...

Here is my Python script:

    import tornado.httpserver
import tornado.websocket
import tornado.ioloop
import tornado.web
import RPi.GPIO as GPIO
import serial
import time

xbee = serial.Serial("/dev/ttyAMA0", 9600)
#xbee.open()

time.sleep(2)

GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)
GPIO.setup(18, GPIO.IN)
GPIO.setup(23, GPIO.IN)
GPIO.setup(24, GPIO.IN)
GPIO.setup(25, GPIO.IN) 

pin18 = GPIO.input(18)
pin23 = GPIO.input(23)
pin24 = GPIO.input(24)
pin25 = GPIO.input(25)

GPIO.setup(18, GPIO.OUT)
GPIO.setup(23, GPIO.OUT)
GPIO.setup(24, GPIO.OUT)
GPIO.setup(25, GPIO.OUT)

class WSHandler(tornado.websocket.WebSocketHandler):

    def open(self):
        print "New connection was opened"
        self.write_message("Connected!")
        self.write_message("pin18 = %s \n pin23 = %s \n pin24 = %s \n pin25 = %s" % (pin18, pin23, pin24, pin25))


    def on_message(self, message):
        global pin18
        global pin23
        global pin24
        global pin25

        if(message == "pin18"):
            print "%s" % message
            if(pin18 == 1):
                GPIO.output(18, GPIO.LOW)
                pin18 = 0
            else:
                GPIO.output(18, GPIO.HIGH)
                pin18 = 1

        elif(message == "pin23"):
            if(pin23 == 1):
                GPIO.output(23, GPIO.LOW)
                pin23 = 0
            else:
                GPIO.output(23, GPIO.HIGH)
                pin23 = 1       

        elif(message == "pin24"):
            if(pin24 == 1):
                GPIO.output(24, GPIO.LOW)
                pin24 = 0
            else: 
                GPIO.output(24, GPIO.HIGH)
                pin24 = 1

        elif(message == "pin25"):
            if(pin25 == 1):
                GPIO.output(25, GPIO.LOW)
                pin25 = 0
            else:
                GPIO.output(25, GPIO.HIGH)
                pin25 = 1

        elif(message == "alloff"):
            GPIO.output(18, GPIO.HIGH)
            pin18 = 1
            GPIO.output(23, GPIO.HIGH)
            pin23 = 1
            GPIO.output(24, GPIO.HIGH)
            pin24 = 1
            GPIO.output(25, GPIO.HIGH)
            pin25 = 1

        elif(message == "allon"):
            GPIO.output(18, GPIO.LOW)
            pin18 = 0
            GPIO.output(23, GPIO.LOW)
            pin23 = 0
            GPIO.output(24, GPIO.LOW)
            pin24 = 0
            GPIO.output(25, GPIO.LOW)
            pin25 = 0

        else:
            self.write_message("%s\n" % message)
            print "%s" % message
            xbee.write("%s" % message)

    def on_close(self):
        print "Connection was closed..."

application = tornado.web.Application([
    (r'/ws', WSHandler),
])

if __name__ == "__main__":
    http_server = tornado.httpserver.HTTPServer(application)
    http_server.listen(9090)
    tornado.ioloop.IOLoop.instance().start()

Can anyone tell me what the problem is? :)