There is a misprint in the Lua API of "enemy" (here: http://www.solarus-games.org/doc/latest/lua_api_enemy.html)
in the last lines of the example of the Overview, it should be written m:start(self) instead of self:start_movement(m).
I suppose that was the old syntax. That misprint could give problems for beginners of solarus.
Nice catch. I just fixed it in 1.4. Thanks for the report!