Source: Question answered while rubber-ducking in the chat-room, LOL ;)
When the Matrix methods try to look for the non-existent methods; they'll error out — naturally — as a NameError.
Note Although there are other great, detailed answers here about idiomatic use of abstract classes, what I eventually went with was just removing the methods; so I think I should accept my answer as per the FAQ and +1 the rest of the useful ones.