Look rotation viewing is zero warning

Sorry for the noob question, and also for my bad english.

I am following a tutorial on how to get started with the basics of unity. I'm at a part of it that is teaching me how to rotate a player according to the direction it is moving, so that it will face said direction. Since the destination of said movement is set to a Vector 3 variable called "currentMovement" , I thought that using "transform.rotation = Quaternion.LookRotation(currentMovement)" would work. It indeed works when the player is moving, but when the player just stays put Unity gives a warning saying "Look rotation viewing is zero" and then the player object starts to glitch (and by glitch I mean starting to constantly alternate between random rotations or something like that). The tutorial said/told that the fix to this would be to use "Quaternion.LookRotation(new Vector3(currentMovement.x, 0, currentMovement.z))" instead of the code I used, since currentMovement.y is set to 0 when the player is on the ground (and said player WAS standing on the groun), and apparentely you can't give a zero value to a look rotation (as indicated by the warning). **But the solution proposed also sets the y axis to 0, so why did it work?**It doesn't make sense to me, since it seens that one value that would end up being set to 0 (currentMovement.y) was replaced by the integer 0 itself...

I'm going to paste my code below:

 public float movSpeed = 5;


 public float rotateSpeed = 90;

 public float jumpSpeed = 10;


 public float gravity = 15;

 public float moveSpeedSmooth = 0.3f;
 public float rotateSpeedSmooth = 0.3f;

 CharacterController controller;
 Vector3 currentMovement;
 Vector3 currentMovementV;
 Transform cameraTransform;

 float verticalSpeed;
 public float currentForwardSpeed;
 public float forwardSpeedV;
 public float targetRotation;
 public float currentRotation;
 public float rotationV;



 void Start()
 {
     controller = GetComponent<CharacterController>();
     cameraTransform = Camera.main.transform;
 }



 void Update()
 {
     Vector3 movementInput = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
     if (movementInput.magnitude > 1)
     {
         movementInput.Normalize();
     }

     Vector3 targetMovement = movementInput;
     targetMovement = cameraTransform.TransformDirection(targetMovement);
     targetMovement.y = 0;
     targetMovement *= movementInput.magnitude;

     currentMovement = Vector3.SmoothDamp(currentMovement, targetMovement * movSpeed, ref currentMovementV, moveSpeedSmooth);

     transform.rotation = Quaternion.LookRotation(new Vector3(currentMovement.x, 0, currentMovement.z));

     if (!controller.isGrounded)
     {
         verticalSpeed -= gravity * Time.deltaTime;
     }
     
     else
     {
         verticalSpeed = 0;
     }

     if (controller.isGrounded && Input.GetButtonDown("Jump"))
     {
         verticalSpeed = jumpSpeed;
     }

     
     currentMovement.y = verticalSpeed;
     
     controller.Move(currentMovement * movSpeed * Time.deltaTime);

 }