如何修复nxt.motor.BlockedException？

Question

我有以下脚本：

#!/usr/bin/env python

import nxt.locator
from nxt.motor import *

def spin_around(b):
    m_left = Motor(b, PORT_B)
    m_left.turn(100, 360)
    m_right = Motor(b, PORT_C)
    m_right.turn(-100, 360)

print("Script Starting")
b = nxt.locator.find_one_brick()
spin_around(b)

我已经安装了pyUSB和libUSB。我使用python spin.py运行该脚本

然而，当我运行脚本时，我得到了以下异常：

Script Starting
Traceback (most recent call last):
  File "spin.py", line 14, in <module>
    spin_around(b)
  File "spin.py", line 8, in spin_around
    m_left.turn(100, 360)
  File "/Library/Python/2.7/site-packages/nxt/motor.py", line 211, in turn
    raise BlockedException("Blocked!")
nxt.motor.BlockedException: Blocked!

为什么会发生这种异常？