How to implement Map with default operation in Scala

class DefaultListMap[A, B <: List[B]] extends HashMap[A, B] {
    override def default(key: A) = List[B]() 
  }

I wan't to create map A -> List[B]. In my case it is Long -> List[String] but when I get key from map that doesn't have value I would like to create empty List instead of Exception being thrown. I tried different combinations but I don't know how to make code above pass the compiler.

Thanks in advance.