Scala unexpectedly not being able to ascertain type for expanded function

Why, in Scala, given:

a = List(1, 2, 3, 4)
def f(x : String) = { x }

does

a.map(_.toString)

work, but

a.map(f(_.toString))

give the error

missing parameter type for expanded function ((x$1) => x$1.toString)