Scala underscore - ERROR: missing parameter type for expanded function

I know there have been quite a few questions on this, but I've created a simple example that I thought should work,but still does not and I'm not sure I understand why

val myStrings = new Array[String](3)
// do some string initialization

// this works
myStrings.foreach(println(_))


// ERROR: missing parameter type for expanded function
myStrings.foreach(println(_.toString))

Can someone explain why the second statement does not compile?

It expands to:

myStrings.foreach(println(x => x.toString))

You want:

myStrings.foreach(x => println(x.toString))

The placeholder syntax for anonymous functions replaces the smallest possible containing expression with a function.