spray - validation error of request parameters doesn't result in status code 400 - badRequest

Question

I have following spray route

      post {
        parameters(('adId.as[String], 'propertyType.as[TypNe])).as(Import){imp =>
          complete {
            adImporterService ! imp
            StatusCodes.Accepted
          }
        }
      }

Custom deserializer for conversion to Java enum of TypNe:

  implicit val propertyTypeDeserializer = new Deserializer[String,TypNe]{
    def apply(s:String):Deserialized[TypNe]={
      Try(TypNe.valueOf(s.toUpperCase)) match {
        case Success(tn) => Right(tn)
        case Failure(ex) => Left(MalformedContent(s"Wrong property type. Accepted values: ${TypNe.values}", Some(ex)))
      }
    }
  }

and Import case class is defined as follows:

  case class Import(adId: String, propertyType:TypNe){
    require(adId!= null && !adId.isEmpty && propertyType!=null,"adId parameter cannot be null or empty and propertyType has to be defined")
  }

The desired and expected behaviour is that IllegarArgumentException thrown is bind to response code 400 - BadRequest. But I am getting response code 500 - Internal Server Error.

I am using a Default RejectionHandler

  class MainService(route: Route) extends HttpServiceActor {
    implicit val system = context.system

    import spray.routing.RejectionHandler.Default

    override def receive: Receive = runRoute(route)
  }

with default ExceptionHandler - which converts NonFatal exception to InternalServer error.

What is the best practice to achieve a desired behaviour?

Creating a custom ExceptionHandler seems to me as a not correct way to go. What I would probably need to achive a conversion of IllegalArgumentException to MalformedQueryParamRejection within a parameters directive.

Can anyone give a hint how to achieve it?

Thx

UPDATE: Tried different approach but didn't work as expected either.

post {
            parameters(('adId.as[String], 'propertyType.as[String])) { (id,pT) =>
              validate(Try(TypNe.valueOf(pT)).isSuccess,s"propertyType $pT is not a valid ${TypNe.values()}")
              validate(id!= null && !id.trim.isEmpty,"adId parameter cannot be null or empty ")
              complete {
                adImporterService ! Import(id, TypNe.valueOf(pT))
                StatusCodes.Accepted
              }
            }
          }