How to implement implicit Json Writes of embedded object in Play Framework 2.x

there are two classes Foo and Bar. Foo contains a field of Bar. The question is, how do I implement an implicit json Writes for class Foo?

Here is the code:

package models

import play.api.libs.json._

case class Foo(id: String, bar: Bar)

object Foo {
  implicit val implicitFooWrites = new Writes[Foo] {
    def writes(foo: Foo): JsValue = {
      Json.obj(
        "id" -> foo.id,
        "bar" -> foo.bar
      )
    }
  }
}

case class Bar(x: String, y: Int)

object Bar {
  implicit val implicitBarWrites = new Writes[Bar] {
    def writes(bar: Bar): JsValue = {
      Json.obj(
        "x" -> bar.x,
        "y" -> bar.y
      )
    }
  }
}

When I try to compile, I get the following error:

No Json deserializer found for type models.Bar. Try to implement an implicit Writes or Format for this type.

I don't understand this compiler error, since I implemented an implicit Writes for models.Bar class. What is the problem here?