RaycastError

I am using a raycast script to place a 2d object in 2d space, but it keeps coming up with 'Object reference not set to an instance of an object' When i use the same line of code in a 2d world with only 3d objects, it works fine.

     void Update () 
     {
         mousePos = Input.mousePosition;
 
         RaycastHit hit;
         Ray ray = Camera.main.ScreenPointToRay (mousePos); // it selects this line
         
         if (Physics.Raycast (ray, out hit, 1000))
         {
             if(hit.transform.tag == "PlaceablePath")
             {
                 for(int i = 0; i < towers.Count; i ++)
                 {
                     if (towers[i].selected == true && Input.GetButtonDown ("Place"))
                     {
                         towers[i].selected = false;
                         GameObject clone = Instantiate(towers[i].towerObject, hit.transform.position, Quaternion.identity) as GameObject;
                     }
                 }
             }
         }
     }