Making the players head face toward the location of the mouse?

I’m making a 2D game in which your a small girl fighting of ghosts. I’m trying to get my character’s head to face to where the mouse is located. Currently I’m using this code:
//You can get the players position on the screen using Camera.WorldToScreenPoint(). Once you have this you can easily compare it to your mouse position, hope this helps:

//var MousePosition = Input.mousePosition;
//var PlayerPosition;
//function Update (){
//if(MousePosition.x < PlayerPosition.x)
  //  transform.rotation = Quaternion.Euler(0,180,0);
//else if(MousePosition.x > PlayerPosition.x)
  //  transform.rotation = Quaternion.Euler(0,0,0);


// LookAtMouse will cause an object to rotate toward the cursor, along the y axis.
// To use, drop on an object that should always look toward the mouse cursor.
// Change the speed value to alter how quickly the object rotates toward the mouse.
// speed is the rate at which the object will rotate
var speed = 4.0;
function Update () {
    // Generate a plane that intersects the transform's position with an upwards normal.

//Edit to show where error is located. (Line 31)

    var playerPlane = new Plane(Vector3.up, transform.position);
    // Generate a ray from the cursor position
    var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
    // Determine the point where the cursor ray intersects the plane.
    // This will be the point that the object must look towards to be looking at the mouse.
    // Raycasting to a Plane object only gives us a distance, so we'll have to take the distance,
    //   then find the point along that ray that meets that distance.  This will be the point
    //   to look at.
    var hitdist = 0.0;
    // If the ray is parallel to the plane, Raycast will return false.
    if (playerPlane.Raycast (ray, hitdist)) {
        // Get the point along the ray that hits the calculated distance.
        var targetPoint = ray.GetPoint(hitdist);
        // Determine the target rotation.  This is the rotation if the transform looks at the target point.
        var targetRotation = Quaternion.LookRotation(targetPoint - transform.position);
        // Smoothly rotate towards the target point.
        transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, speed * Time.deltaTime);
        // Move the object forward.
        transform.position += transform.forward * speed * Time.deltaTime;

But I’m getting this error constantly:

NullReferenceException UnityEngine.Camera.ScreenPointToRay (Vector3 position) (at C:/BuildAgent/work/812c4f5049264fad/Runtime/ExportGenerated/Editor/UnityEngineCamera.cs:291) HeadRotation.Update () (at Assets/HeadRotation.js:31)

I’m not sure on what the error means, nor what I can do about it. Any help would be appreciated greatly.
Thanks in advance.

My guess is that the error is caused by trying to do a Raycast() with a 0 distance. Perhaps you want something like:

var hitdist = 10.0;
var hit : RaycastHit;
if (playerPlane.Raycast (ray, hit, hitdist)) {
      var targetPoint = hit.point;

You may not want to use hitdist at all (i.e. the maximum distance to project the ray). Note his addresses the error. As for the algorithm for the rotation, it may not give you want you want. There are other ways to deal with 2D rotation.