Pattern match abstact type

trait Aggregate {
    type Command
}

class AggregateHandler(a: Aggregate) {
   def receiveCommand: Receive = {
     case a.Command => ???
   }
}

How can I pattern match on a.Command? I am getting; abstract type pattern AggregateHandler.this.a.Command is unchecked since it is eliminated by erasure and The outer reference in this type test cannot be checked at run time.

How can I workaround this?