I paste the issue open : the primitive method #decompress:fromByteArray:at: aren't well-formed, cause the code of this method is using by VMMaker to generate source code of the primitive, then it can happen which a selector using in the method was never implemented. Then if the code is fix VMMaker may not be able to generate a clean source code. the main point of this issue is the selector call: #primitiveFail, he doesn't exist in the Pharo environment and seem be change by #primitiveFailed which exist, and then the mistake can't be see if we don't try to generate to use VMMaker without using MiscPrimitivesPlugin. I post a fix in the Inbox. i have simply correct #primitiveFailed to #primitiveFail If you have a better solution i will be glad to discuss with you. link : http://code.google.com/p/pharo/issues/detail?id=1437