Well, in many typed languages, a lookup on a map will return an option type. If the key is in the map, it returns Some(value), but if it isn't, it returns None.

Well, in many typed languages, a lookup on a map will return an option type. If the key is in the map, it returns Some(value), but if it isn't, it returns None. By encoding the possibility of a missing key in the return type, you force the programmer to deal with it in the program. So in fact, having a good type system can help deal with those kinds of errors as well.