provide slot value for specific slot in response and resume the conversation

I am working on lex and want to give slot value in the response which will only be asked if the user enters specific input in the previous slot value. I am trying something but I don't I am doing right or not.

i have following slots in lex.

1. Departure_city
2. Arrival_city
3. Departing (oneway or roundtrip)
4. ReturnDate
5. Date (DepartureDate)
6. Flight Schedule

e.g. if user select Roundtrip then ask for the return date otherwise skip that slot and continue the flow by asking value of the remaining of slots

here is the piece of code that I am doing to fulfill this scenario.

"use strict";

const lexResponses = require("./lexResponse");

const depart = ["one-way", "oneway"];

const buildValidationResult = (isValid, violatedSlot, messageContent) => {
  if (messageContent == null) {
    return {
      isValid: isValid,
      violatedSlot: violatedSlot,
    };
  }
  return {
    isValid: isValid,
    violatedSlot: violatedSlot,
    message: { contentType: "PlainText", content: messageContent },
  };
};

function validateBookaflight(
  Departing,
  ReturnDate
) {
  if (Departing && depart.indexOf(Departing.toLowerCase()) === -1) {
   
     return {
          dialogAction: {
            type: "ElicitSlot",
            intentName: "Bookaflight",
            slots: {
              Departure_city: Departure_city,
              Arrival_city: Arrival_city,
              Departing: Departing,
              ReturnDate: ReturnDate,
            },
            slotToElicit: "ReturnDate",
            message: {
              contentType: "PlainText",
              content: "Please enter return date,(yyyy-mm-dd)",
            },
          },
        }
    };
     return buildValidationResult(true, null, null);
}

function buildFulfilmentResult(fullfilmentState, messageContent) {
  return {
    fullfilmentState,
    message: { contentType: "PlainText", content: messageContent },
  };
}

error:

An error has occurred: Invalid Lambda 
Response: Received invalid response from 
Lambda: Can not construct instance of 
ElicitSlotDialogAction, problem: 
slotToElicit must not be blank in ElicitSlot 
dialog action at 
[Source: {"sessionAttributes":{},"dialogAction":{"type":"ElicitSlot","intentName":"Bookaflight",
"slots":{"ReturnDate":null,"Departure_city":"london","Flight_schedule":null,"Arrival_city":"lahore","Date":null,
"Departing":"roundtrip",
"undefined":null}}}; line: 1, column: 241]

please tell what i am doing wrong or if you have any issue understanding my requirement.

from provide slot value for specific slot in response and resume the conversation