Drag Object with Mouse Along Y-Axis

I am trying to create a pong type game and I have ran into a problem. I want the player to be able to drag the paddle along the y-axis but the game object is not behaving the way I want it to. Here is my code:

     private Vector3 screenPoint;
     private float startPosX;
     private float startPosZ;
 
     void Update ()
     {
         startPosX = -10.0f;
         startPosZ = 0f;
 
         transform.position = new Vector3(startPosX, Input.mousePosition.y, startPosZ);
     } 
 
     void OnMouseDown()
     {
         screenPoint = Camera.main.WorldToScreenPoint(gameObject.transform.position);
     }
 
     void OnMouseDrag()
     {
         Vector3 currentScreenPoint = new Vector3(startPosX, Input.mousePosition.y, screenPoint.z);
         Vector3 currentPos = Camera.main.ScreenToWorldPoint(currentScreenPoint);
         transform.position = currentPos;
     }

I only want the paddle to move along the Y-Axis but it seems that when I try to drag the object it suddenly appears off the game screen. Is there another way to do something like this? Or am I doing this wrong?