[Discuss-gnuradio] import qtgui error

I tried the code of 'Tutorial - Using the QT GUI Blocks in GNU Radio', but error occurred.

Traceback (most recent call last):
  File "qt_basics.py", line 4, in <module>
    from gnuradio.qtgui import qtgui
  File "/opt/gnuradio/3.3.0/lib/python2.6/site-packages/gnuradio/qtgui/qtgui.py", line 6, in <module>
    import _qtgui
ImportError: /opt/gnuradio/3.3.0/lib/libgnuradio-qtgui-3.3.0.so.0: undefined symbol: _ZN9QwtPicker5resetEv

Who can tell me what is the problem? Thank you!