NSTableview datasource and NSPopupButtonCell

For some reason none of my posts to the cocoa-dev list are getting through so here's question for the forum.

I have a NSTableview with a datasource consisting of an NSArray of
dictionaries. One column in my table has an NSPopupbuttonCell. The
content and contentValues are bound to a Core Data "Account" object.
I use a "name" property as my contentValue.

I'm having trouble changing the popup button. I know in my datasource
I get an index of the selected object and in my setObjectValue
datasource method I set the key in the dictionary with this new
NSNumber intValue. However, in operation when I change the popupbutton
it very quickly changes back to the first object in the list (index
0).

Clearly I'm missing something or have set something wrongly in IB.