Переназначение значения val в Scala

I am doing a training exercise in Scala and getting this val reassignment error. I don't see where I am reassigning a new value to a val

class personTest
{
  val alf = Person("Alf", 30, List(EmailAddress("alf.kristian@gmail.com")))
  val fredrik = Person("Fredrik", 33, List(EmailAddress("fredrik@vraalsen.no"), EmailAddress("fvr@knowit.no")))
  val johannes = Person("Johannes", 0, Nil)

  val persons = List(alf, fredrik, johannes)

   @Test
  def testNameToEmailAddress
  {
    // Create a map from each persons name to their e-mail addresses,
    // filtering out persons without e-mail addresses
    // Hint: First filter list, then use foldLeft to accumulate...
    val emptyMap: Map[String, List[EmailAddress]] = Map()

    val nameToEmail = persons.filter(_.emailAddresses.length>0).foldLeft(emptyMap)((b,p)=> b+=p.name->p.emailAddresses)

    assertEquals(Map(alf.name -> alf.emailAddresses, fredrik.name -> fredrik.emailAddresses), nameToEmail)
  }

}

and I am getting this error

error: reassignment to val
val nameToEmail = persons.filter(_.emailAddresses.length>0).foldLeft(emptyMap)((b,p)=> b+=p.name->p.emailAddresses)