wxpython，dc.GetAsBitmap返回无效位图

我试图得到我的dc绘制的位图，但当我将其转换为图像时，我得到了一个位图

  File "C:\Python25\lib\site-packages\wx-2.8-msw-unicode\wx\_gdi.py", line 618, in ConvertToImage
    return _gdi_.Bitmap_ConvertToImage(*args, **kwargs)
wx._core.PyAssertionError: C++ assertion "bmp.Ok()" failed at ..\..\src\msw\dib.cpp(148) in wxDIB::Create(): wxDIB::Create(): invalid bitmap

当试图将所述位图绘制到另一个dc时，也会出现相同的错误。所以我记得我在这里读到过一种方法，将ClientDC的图形存储到MemoryDC中，然后清理ClientDC并将MemoryDC的内容重新绘制到它上面。怎么做

bitmap = wx.EmptyBitmap(width, height)

memory = wx.MemoryDC()
memory.SelectObject(bitmap)
#set pen, do drawing.
memory.SelectObject(wx.NullBitmap)

img = wx.ImageFromBitmap(bitmap)