Pattern matching BigInts

This factorial implementation works for numbers up to a certain size: ``` def factorial(n:Int):Int = n match { case 0 => 1 case x => x * factorial(x - 1) } ``` I tried to use BigInt to make it work for numbers of any size like this ``` val zero = BigInt(0) def factorial(n:BigInt):BigInt = n match { case zero => 1 case x => x * factorial(x - 1) } ``` Every call to factorial comes back with 1 regardless of the value of n. I assumed this is because the first case is always matching, and proved it is so by changing it to ``` case zero => 22 ``` and verifying that 22 was returned for every input. So my two questions are - Why is the first case always matching? - Is there a way to get a BigInt version of this function to work whilst sticking to pattern matching?