Mocking in scala generates a java.lang.NoSuchMethodException

Hey I am trying to test the following class :

Class Foo {
 def f: Int = 4+g()

 def g: Int = 2
}

My test is the following:

class FooSpec extends PlaySpec with MockFactory {
  val foo = new Foo()
  "Foo" must {
    "Call function f" in {
        (foo.g())
        .expects()
        .once()
        .returns(5)

        foo.f must be (9)
     }
   }
}

My test is failing saying that :

java.lang.NoSuchMethodException: Foo.mock$g$0()
java.lang.Class.getMethod(Unknown Source)
...

I am not sure to see why ...

I am using scalatest and scalamock :

"org.scalatestplus.play" %% "scalatestplus-play" % "1.5.0" % "test"
"org.scalamock" %% "scalamock-scalatest-support" % "3.2.2" % "test"