[Matplotlib-users] pcolor and masked arrays; am I missing something?

Howdy,

I'm a recent refugee from GMT (Generic Mapping Tools) and am very happy to
have found matplotlib.

I've been having one nagging issue however that I must resolve as I require
this ability. Basically, I need to mask 2d arrays and plot the result with
pcolor via basemap.

>From the documentation it seems this should be fairly straight forward and
I'm hoping that this is the case.

Here is an example of problem.
-------------------------------------------
from pylab import *
import matplotlib.numerix.ma as ma # matplotlibrc has numerix : numpy
from matplotlib.toolkits.basemap import Basemap, shiftgrid, addcyclic
import numpy as N
.
.
.

im = 144
jm = 93
lons  = N.array([0., ...  357.5],dtype=float)
lats =  N.array([-90., ... 90.],dtype=float)
topo_screen = N.zeros((jm,im),dtype='float')
topo_screen[10] = 1
new_test = 100.*N.ones((jm,im),dtype='float')
masked_test = ma.masked_where(topo_screen,new)

This works and a screen dump of masked_test seem correct.

A call to pcolor however results in the following:
matplotlib version 0.90.1
verbose.level helpful
interactive is False
units is True
platform is linux2
numerix numpy 1.0.1
.
.
.
/usr/local/lib/python2.5/site-packages/numpy/core/ma.py:604: UserWarning:
Cannot automatically convert masked array to numeric because data is masked
in one or more locations.
-------------------------------------------

Is this an embarrassingly simple error on my part? I hope so. Any help would
be much appreciated.

Mike