++ on Set[Any] doesn't compile

Hi. I receive a compile error when I try to do this:   val s1 = Set[Any](1, 2, 3) val s2 = Set[Any]("a", "b") s1 ++ s2   The error I receive is:

error: ambiguous reference to overloaded definition,
both method ++ in trait Addable of type (xs: scala.collection.TraversableOnce[Any])scala.collection.immutable.Set[Any]
and  method ++ in trait TraversableLike of type [B >: Any,That](that: scala.collection.TraversableOnce[B])(implicit bf: scala.collection.generic.CanBuildFrom[scala.collection.immutable.Set[Any],B,That])That
match argument types (scala.collection.immutable.Set[Any])
       s1 ++ s2
       ^   What am I doing wrong?  I must admit the error message is a bit difficult to understand...

Henri