nil just always comes back with 0 or nil or whatever -- it's like a really dumb version of None in the Maybe monad, and trying to get you some of the convenience (except way, way worse from a safety perspective).

For extra fun, NSNull (the "designated" object (as opposed to an actual zero value of nil) to represent nil) doesn't happily respond to any message sent to it -- i.e., if you replace nil with NSNull somewhere in the code, it will most likely blow up.