python3 winpexpect问题请教

代码如下（前置条件为：  windows系统  python3.4   winpexpect版本是1.5）为了验证，所以简化代码，expect匹配都去掉了。
import winpexpect
import sys

hostip = '172.18.24.1'
print ('00')
hostcmd = 'telnet ' + hostip
print ('001')
child = winpexpect.winspawn(hostcmd)
print ('002')
username = 'root'
time.sleep(1)

#username = username.encode('utf-8')

child.sendline(username)
print ('1')

time.sleep(1)

child.sendline('123456')

child.logfile = sys.stdout


运行结果是
>>> ================================ RESTART ================================
>>>
00
001
002
Traceback (most recent call last):
  File "C:\Users\A103\Desktop\pex2.py", line 17, in <module>
    child.sendline(username)
  File "C:\Python34\lib\site-packages\winpexpect-1.5-py3.4.egg\pexpect.py", line 961, in sendline
    n = self.send(s)
  File "C:\Python34\lib\site-packages\winpexpect-1.5-py3.4.egg\pexpect.py", line 953, in send
    c = os.write(self.child_fd, s.encode('ascii'))
OSError: [Errno 22] Invalid argument
>>>


尝试编码转换过，无效（例如username = username.encode('utf-8')或者decode('')等等

请教各位大神，如何解决。