[Pharo-users] Fuel FLMethodChanged error

I'm trying to serialize and materialize some domain objects. If I run this: FLSerializer serialize: users toFileNamed: 'users.fuel'. The file users.fuel is created. If in the same workspace in the same Pharo 1.3 image on Cog 2550 / Linux 64 bit I subsequently run: FLMaterializer materialzeFromFileNamed: 'users.fuel' I get an FLMethodChanged error. If I just serialize and materialize strings, numbers, and DateAndTime objects to files everything works fine and all 248 Fuel tests pass no problem. The incoming method it has a problem with is an #initialize method. The serializeHash is 15407 and the result of 'method bytecodesHash' is 30998. What should I check to diagnose/fix this problem? Thanks