type errors on the solution of `5.5.4.3 Sequencing Computaঞons`

the last challenge is this one :

return a List[Int] containing both all the elements and their negaঞon. Order is not important. Hint: Given
an element create a list containing it and its negaঞon.
See the soluঞon

I solved it this way :

sealed trait Maybe[A] {
  def flatMap[B](fn: A => Maybe[B]): Maybe[B] =
    this match {
      case Full(v) => fn(v)
      case Empty() => Empty[B]()
    }

}

final case class Full[A](value: A) extends Maybe[A]
final case class Empty[A]() extends Maybe[A]

val list = List(Full(3), Full(2), Full(1))

list.map(maybe => maybe flatMap { x => if(x % 2 == 0) Full(x) else Empty[Int]() })

where as the given solution is this one :

list.map(maybe => maybe flatMap { x => if(x % 2 == 0) Full(x) else Empty() })

but that one gives this error :

Error:(15, 135) type mismatch;
 found   : A$A50.this.Empty.type
 required: A$A50.this.Maybe[?]
def get$$instance$$res0 = /* ###worksheet### generated $$end$$ */ list.map(maybe => maybe flatMap { x => if(x % 2 == 0) Full(x) else Empty })

So did I understand something wrong or is the given solution wrong ?