[Zope] zclass instance "broken" on copy/paste?

[Darran Edmundson]

I have a ZClass (Shot) which is nested beneath
a container ZClass (Storyboard).  I.e., an
instance of Storyboard might be:

Scene1 (Storyboard)
 - shot1 (Shot)
 - shot2 (Shot)
 - shot3 (Shot) ...

For reasons that I cannot discern, copying and pasting 
instances of Shot does not work.  I can copy them, but 
pasting creates a broken object:

  "This object is broken because the unknown product that 
   created it is no longer installed or is installed 
   incorrectly."

I suspect the copy has in fact worked because the title
in brackets is that of the source object.  This is an 
isolated problem on my site.  All other nested Zclasses 
work fine but I cannot pin down the difference.  This is 
Zope 2.1.4.

Has anyone else encountered this problem?

Darran.