Initializing a scala BitSet from a range

I'd like to initialize a scala BitSet to contain the integers from 1 to N. The following will work, but I'm looking for a better solution:

var s = BitSet.empty ++ (1 to n)

I was hoping that I could do something like this:

var s:BitSet = (1 to n).toSet

...but that results in an error:

error: polymorphic expression cannot be instantiated to expected type;
  found   : [B >: Int]scala.collection.immutable.Set[B]
  required: scala.collection.immutable.BitSet

Am I missing something obvious?