can't debug TinyBlog in the Pharo MOOC

hi guys I'm following the Pharo MOOC and I'm at Week 4. the other day, I was trying to implement TinyBlog: Data Persistency using Voyage and Mongo, when I tried to compile the code, I've got an Error. the title of Error says: "MessageNotUnderstood: receiver of "save:" is nil", and it refers to TBMBlog>>writeBlogPost. TBMBlog>>writeBlogPost: aTBMPost posts add: aTBMPost. aTBMPost save. self save . thanks for your answers! -- Sent from: http://forum.world.st/Pharo-Smalltalk-Users-f1310670.html