Tested both in Pharo-Core & Squeak trunk image. The class i defined is: ProtoObject subclass: #BBBNameOfSubclass instanceVariableNames: '' classVariableNames: '' poolDictionaries: '' category: 'NILSuperClass'. BBBNameOfSubclass superclass: nil In attachment you can fin the fileout of it. And MC package, saved with it. If you try load the package, MC will display warning that you should resolve dependencies 'nil' before loading the class ;) Background: I wanted to migrate to newer image and saved my package, im working on, which i didn't touched for 3-4 months, and found that it can't be loaded :( -- Best regards, Igor Stasenko AKA sig.