Object doesn't rotate well with camera raycasting.

Hi, I'm a beginner in unity and I'm working on my first project after doing a few tutorial projects like Space Shooter, Tanks! etc.

I have a tower with a turret on it and I want to move - rotate - the turret along the X,Y(around the Z axis) plane with raycasting from the main camera. My LayerMask("Screen") is the quad I used as the background.

The code:

 using UnityEngine;
 using System.Collections;
 
 public class TurretRotation : MonoBehaviour
 {
     private Rigidbody rb;
     private int screenMask;
     private float camRayLength = 100f;
 
     
     void Awake ()
     {
         rb = GetComponent<Rigidbody>();
         //sets the background as the layer for raycasting
         screenMask = LayerMask.GetMask("Screen");
     }
     
     void FixedUpdate ()
     {
         Aim();
     }
 
     void Aim()
     {
         Ray camRay = Camera.main.ScreenPointToRay(Input.mousePosition);
         RaycastHit screenHit;
         if (Physics.Raycast(camRay, out screenHit, camRayLength, screenMask))
         {
             Vector3 turretToMouse = screenHit.point - transform.position;
             turretToMouse.z = 0.0f;
             //in order to use a vector 3 for rotation, it must be converted to a quaternion
             Quaternion newRotation = Quaternion.LookRotation(turretToMouse);
             rb.MoveRotation(newRotation);
         }
     }
 }
 

When I test it and point the mouse over the screen the turret is either facing the camera or facing the background.