Scala, regex, and null

One of the perks of using scala is that I hardly ever see a NullPointerException unless I'm working with java libraries. The primary reason is because most scala libraries tend to use Option rather than null. However, while using a regex with pattern matching I was surprised by a NullPointerException when trying to look at the result of an optional capture group. Consider the following example:

scala> val Pattern = "(a)(b)?".r
Pattern: scala.util.matching.Regex = (a)(b)?

scala> "a" match { case Pattern(a, b) => printf("[%s][%s]%n", a, b) }
[a][null]

scala> "ab" match { case Pattern(a, b) => printf("[%s][%s]%n", a, b) }
[a][b]

I just assumed that b would be of type Option[String]. There is probably a good reason for this travesty, my guess would be something about making it work with the type system, but after using scala for a while it just seems wrong to be getting a null value.