[Matplotlib-users] odd "singular matrix" error

I have no idea what this is. If I create a certain plot first in an
application, it throws this error (edited to the key part):

Traceback (most recent call last):

  File "thescript.py", line 2147, in AddPatchBar
    ax.add_patch(patch)
  File "C:\Python25\lib\site-packages\matplotlib\axes.py", line 1471, in
add_patch
    self._update_patch_limits(p)
  File "C:\Python25\lib\site-packages\matplotlib\axes.py", line 1492, in
_update_patch_limits
    self.transData.inverted())
  File "C:\Python25\lib\site-packages\matplotlib\transforms.py", line 1954,
in inverted
    return CompositeGenericTransform(self._b.inverted(), self._a.inverted())
  File "C:\Python25\lib\site-packages\matplotlib\transforms.py", line 1448,
in inverted
    self._inverted = Affine2D(inv(mtx))
  File "C:\Python25\lib\site-packages\numpy\linalg\linalg.py", line 445, in
inv
    return wrap(solve(a, identity(a.shape[0], dtype=a.dtype)))
  File "C:\Python25\lib\site-packages\numpy\linalg\linalg.py", line 328, in
solve
    raise LinAlgError, 'Singular matrix'
numpy.linalg.linalg.LinAlgError: Singular matrix

But, the odd part is that if I create a completely different and totally
separate plot *before* this one, and *then* I try to plot this one, I do
not get this error and this plot shows fine. That makes no sense to me. Or
also, if I plot this "patch" on a plot with a few other lines plotted, it
also works.

Does anyone have any idea what could be causing this?  Thanks.