What to set as default when the type is A (Scala)

I have an exercise in Scala in which I have to transform that kind of list (a,a,a,b,c,d,d,e,a,a) into ((a,3),(b,1),(c,1),(d,2),(e,1),(a,2)). I obviously know that my algorithm is not correct yet, but I wanted to start with anything. The problem is that I don't know how to turn on the function (last line), because the error is that whatever I take as previous' argument, it says that required: A, found: Int/String etc. The previous was meant to be as a head of the previous iteration.

def compress[A](l: List[A]): List[(A, Int)] = {
def compressHelper(l: List[A], acc: List[(A, Int)], previous: A, counter: Int): List[(A, Int)] = {
  l match {
    case head::tail => {
      if (head == previous) {

        compressHelper(tail, acc :+ (head, counter+1), head, counter+1)
      }
      else {
        compressHelper(tail, acc :+ (head, counter), head, 1)
      }

    }
    case Nil => acc



  }


}
compressHelper(l, List(), , 1)

}