Possible bug in imshow?

if I run:

PL.imshow(TArm,cmap = ML.cm.winter )

I get the error listed at the end of this message:

However if I run:
PL.imshow(TArm,cmap = ML.cm.winter,vmin=-50.,vmax=50.)

Then all goes as expected - from where the code goes astray, it appears as if the autoscale limits are not being set???which is
circumvented if a specify the max and min explicitly.

Is this a problem? or am I missing something?


/System/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/site-packages/matplotlib/pylab.py in colorbar(tickfmt)
     597 N = 200
--> 599 c = linspace(cmin, cmax, N)
     600 C = array([c,c])

/System/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/site-packages/matplotlib/mlab.py in linspace(xmin, xmax, N)
      73 def linspace(xmin, xmax, N):
      74 if N==1: return xmax
---> 75 dx = (xmax-xmin)/(N-1)
      76 return xmin + dx*arange(N)

TypeError: unsupported operand type(s) for /: 'array' and 'int'