Why isn't operator overloading handling this?

The following bit of code:

case class CharSet(set:String) extends Matcher("[" + set + "]") {

/** Char class subtraction; things in this but not in 'notin' */
def -(notin:CharSet):FinalCharClass = FinalCharClass(set, true,
notin.set)
def -(notin:String):CharSet = this - CharSet(notin)
...
}

results in a compiler error:

Macintosh-5:scalaapplication1 Ken$ fsc -deprecation rex.scala
/Users/Ken/NetBeansProjects/ScalaApplication1/src/scalaapplication1/
rex.scala:128: error: type mismatch;
found : rex.CharSet
required: String
def -(notin:String):CharSet = this - CharSet(notin)
^
one error found

where the "^" points to the CharSet(notin) expression. If I comment
out the second '-' def, everything
works fine, not surprisingly.

But isn't this a case where operator overloading should be able to
determine which version of '-'
to call? What am I missing?

Thanks,
Ken