wx.Grid -- SetCellHighlightColor bomb

What could cause this traceback? I'm running Python 2.5.1 and wxPython 2.8.4 under Windows XP SP2.

The line of code that triigers the traceback: self.SetCellHighlightColour('BLUE')

The traceback:

   File "C:\Python25\Lib\site-packages\wx-2.8-msw-unicode\wx\grid.py", line 1519, in SetCellHighlightColour
     return _grid.Grid_SetCellHighlightColour(*args, **kwargs)
wx._core.PyAssertError: C++ assertion "uiIndex < m_nCount" failed at ..\..\include\wx/dynarray.h(813) in wxBaseArrayInt::Item()

Bob