errors trying to use axes3d

Hi all,

I've got the latest version of matplotlib (0.90) installed on my mac with the latest scipy and numpy. I'm trying to use matplotlib's 3D plotting functionality interactively from ipython on OS X (Intel Mac) with the TkAgg backend. Regular 2D plotting works just fine, but when I create an Axes3D instance using a pylab.figure instance (following the Scipy cookbook mplot3D page), as soon as I click in the Tk window a raft of error messages pops up in the ipython console. There seem to be several of the following errors occurring wherever matrix multiplication is involved:

   File "/Library/Frameworks/Python.framework/Versions/2.4/lib/python2.4/site-packages/matplotlib/", line 185, in proj_transform_vec
     vecw = nx.matrixmultiply(M,vec)
TypeError: unsupported operand type(s) for *: 'NoneType' and 'float'
Exception in Tkinter callback

The stack traces for these errors are very long, so I haven't included them. Am I missing some simple configuration detail to get 3D plotting working?