RaycastHit2D not interacting with tags

I've been hacking my way through references and other questions, but I haven't stumbled upon an answer that has helped a whole ton. I've tried to create a ray that projects from the position of the mouse, and that ray is supposed to 'pick up' any tags that it collides with. On line 19 (not including the part that wasn't transmitted into the code), it says "NullReferenceException: Object reference not set to an instance of an object" and I'm quite new to unity, so I'm not entirely sure how to interpret this... If anyone could help with my little blip, that would be greatly appreciated!

using UnityEngine; using System.Collections;

public class MousePosition : MonoBehaviour {

 private Vector2 ScreenPosition;

 public bool MouseOnTent = false;
  
 void Start () {
 }

 void Update () {
     MouseInteraction ();
 }

 void MouseInteraction() {
     ScreenPosition = camera.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, 0));
     RaycastHit2D hit = Physics2D.Raycast (ScreenPosition, Vector2.zero);
     //Creates a ray that detects if it is hitting a tag where the mouse is


     if (hit != null) {
         if (hit.transform.tag == "Tent")
             MouseOnTent = true;
     }
     else {
         MouseOnTent = false;
     }
 }

}