masking values in quiver plot


maybe I've misunderstood something concerning quiver plots:
I want to exclude some data from a quiver plot. Here's an example:

import numpy as N
import pylab as pl
import as ma

# prepare data
X,Y = pl.meshgrid(range(5),range(5))
angles = pl.rand(5,5)
U = N.cos(angles)
V = N.sin(angles)

# prepare mask
M = N.zeros((5,5), dtype='bool')
M[2,2] = True

# apply mask
Um = ma.masked_array(U,mask=M)
Vm = ma.masked_array(V,mask=M)

# plot
# pl.plot(range(5),Um[2],'x')

Using the commented 'plot' command works as expected, but 'quiver' results in empty axes. If I take U,V instead of Um,Vm, the arrows are drawn ..
What am I doing wrong or is there another way to exclude arrows from being plotted?

  Thank you in advance,