How can I stop player snapping back to 0,0,0 rotation when I release keys?

My player moves with my keys properly, but when I release the keys he snaps back to pointing directly upwards. How can I stop this?

(Script placed on cube with Character Controller):

 using UnityEngine;
 using System.Collections.Generic;
 using System.Collections;
 
 public class PlayerControl : MonoBehaviour
 {
 
     public float speed = 9.0F;
     public float gravity = 13.0F;
 
     private Vector3 moveDirection = Vector3.zero;
     public CharacterController controller;
 
     void Start()
     {
         // Store reference to attached component
         controller = GetComponent<CharacterController>();
     }
 
     void Update()
     {
 
         // Character is on ground (built-in functionality of Character Controller)
         if (controller.isGrounded)
         {
 
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             transform.GetChild(0).rotation = Quaternion.LookRotation(moveDirection);
 
             moveDirection *= speed;
 
         }
 
        //GRAVITYYY
         moveDirection.y -= gravity * Time.deltaTime;
         controller.Move(moveDirection * Time.deltaTime);
 
     }
 }