Calling a function on an iso

iso.pony

class Foo
  var n: USize = 0
  new iso create() => None
  fun iso set_n( m: USize ) =>
    n = m

actor Main
  var foo: Foo iso
  new create( env: Env ) =>
    foo = Foo()
    foo.set_n(3)

This gets error
Error:
/home/pd/Develop/PonyLang/t/t.pony:11:14: receiver type is not a subtype of target type
    foo.set_n(3)
             ^
    Info:
    /home/pd/Develop/PonyLang/t/t.pony:11:5: receiver type: Foo iso!
        foo.set_n(3)
        ^
    /home/pd/Develop/PonyLang/t/t.pony:4:3: target type: Foo iso
      fun iso set_n( m: USize ) =>
      ^
    /home/pd/Develop/PonyLang/t/t.pony:8:12: Foo iso! is not a subtype of Foo iso: iso! is not a subtype of iso
      var foo: Foo iso