PyObjC with Xcode 3.0 Leopard

[flyfree]

I got an error during making a python application with xcode 3.0 in OS
X Leopard.

(KeyError: 'NSUnknownKeyException - [<NSObject 0xd52fe0>
valueForUndefinedKey:]: this class is not key value coding-compliant
for the key calculatedMean.')

The application is a simple example of how to use the PyObjC with
xcode 2.0.
http://developer.apple.com/cocoa/pyobjc.html

Could you give any hint to start to dig into it?