It looks like that the workingCopiesFromConfiguration gives a collection containing a nil val I copied the collection in to a temp var so that I could debug (workingCopies) When I do: workingCopies select: [ :each | each isNil ] --> an OrderedCollection(nil nil nil) Uh Oh... I've made some changes and got something that looked like "proper". I saved it here. http://www.smalltalkhub.com/#!/~philippeback/HOWebStack/packages/Snapshotcel... My use case was: Snapshotcello new configurationClass: ConfigurationOfHOWebStack; sourceVersion: #development; targetVersion: '0.3-snapshot'; snapshot. With the configuration http://www.smalltalkhub.com/#!/~philippeback/HOWebStack/packages/Configurati... version .14 Awesome tool for sure. Phil