Unable to Plot Date Axis Due To "TypeError: a float is required"

I'm getting a nonsensical error when attempting to make a simple plot
with a date axis. Googling shows this similar errors, but none with a
date axis. What could be causing this?

import datetime
from pylab import figure, show
fig = figure()
ax = fig.add_subplot(111)
dates = [datetime.date(2008, 1, 30),datetime.date(2008, 1, 31)]
values = [10.25, 10.0]
ax.plot_date(dates, values)#, '-')

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/matplotlib/axes.py", line
2585, in plot_date
    ret = self.plot(x, y, fmt, **kwargs)
  File "/usr/lib/python2.5/site-packages/matplotlib/axes.py", line 2535, in plot
    for line in self._get_lines(*args, **kwargs):
  File "/usr/lib/python2.5/site-packages/matplotlib/axes.py", line
428, in _grab_next_args
    for seg in self._plot_3_args(remaining, **kwargs):
  File "/usr/lib/python2.5/site-packages/matplotlib/axes.py", line
404, in _plot_3_args
    func(x[:,j], y[:,j])
  File "/usr/lib/python2.5/site-packages/matplotlib/axes.py", line
385, in makeline
    axes=self.axes,
  File "/usr/lib/python2.5/site-packages/matplotlib/lines.py", line
226, in __init__
    self.set_data(xdata, ydata)
  File "/usr/lib/python2.5/site-packages/matplotlib/lines.py", line
315, in set_data
    self.recache()
  File "/usr/lib/python2.5/site-packages/matplotlib/lines.py", line
320, in recache
    x = ma.asarray(self.convert_xunits(self._xorig), Float)
  File "/usr/lib/python2.5/site-packages/numpy/core/ma.py", line 2122,
in asarray
    return array(data, dtype=dtype, copy=0)
  File "/usr/lib/python2.5/site-packages/numpy/core/ma.py", line 573,
in __init__
    self._data = c.astype(tc)
TypeError: a float is required