Match.Optional for null

Securing my Meteor app, I came across a strange quirk of Match.Optional from the “check” package. Here an example in CoffeeScript:

Meteor.methods
  test: (x) -> check x, Match.Optional String

I expected the match to be successful when calling the method like this:

Meteor.call 'test', undefined

But instead, I got a Match failed exception. After debugging for a bit, it turned out that the undefined becomes a null when it arrives on the server. Which is odd, of course—but also annoying as the check x, Match.Optional String fails for null (only undefined or String are acceptable). To overcome this, I had to write up my own variation of Match.Optional. Here is the new Match.OptionalOrNull that does the trick:

Match.OptionalOrNull = (pattern) -> Match.OneOf undefined, null, pattern

The name is lousy, okay. An alternative would be to override the original method, but that’s bad practice of course. But apart that I keep wondering: what’s the purpose of making Match.Optional fail on null in the first place?