runtime error

Could anyone explain why I get a runtime error when using pylab.ion? (see below)

I am running the latest version of matplotlib on Python 2.5 (windows)

Thanks,

Daniel Fish

#-----------Offending code--------

import pylab

pylab.ion()
pylab.plot(range(10))

···

#-------------------------------------

Error Message:

Runtime Error!

Program: C:\Python25\pythonw.exe