mnemosyne error using python2-pyqt4

An unexpected error has occurred.
Please forward the following info to the developers:

Traceback (innermost last):
  File "/usr/bin/mnemosyne", line 191, in <module>
    debug_file=options.debug_file)
  File "/usr/lib/python2.7/site-packages/mnemosyne/libmnemosyne/__init__.py", line 172, in initialise
    self.register_components()
  File "/usr/lib/python2.7/site-packages/mnemosyne/libmnemosyne/__init__.py", line 242, in register_components
    exec("from %s import %s" % (module_name, class_name))
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/mnemosyne/pyqt_ui/statistics_wdgts_plotting.py", line 9, in <module>
    from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
  File "/usr/lib/python2.7/site-packages/matplotlib/backends/backend_qt4agg.py", line 17, in <module>
    from .backend_qt5agg import NavigationToolbar2QTAgg
  File "/usr/lib/python2.7/site-packages/matplotlib/backends/backend_qt5agg.py", line 18, in <module>
    from .backend_qt5 import QtCore
  File "/usr/lib/python2.7/site-packages/matplotlib/backends/backend_qt5.py", line 32, in <module>
    from .qt_compat import QtCore, QtGui, QtWidgets, _getSaveFileName, __version__
  File "/usr/lib/python2.7/site-packages/matplotlib/backends/qt_compat.py", line 91, in <module>
    from PyQt5 import QtCore, QtGui, QtWidgets
 ImportError: No module named PyQt5