Positioning a instantiated object using Input.Mouseposition doesn't position right?

So, I'm trying to instantiate a object on mouse position, but the object is instantiated on a totally different position when launching the game.

Here's my current code :

     void Update()
     {
         if (towerBools [0] == true) {
             if (Input.GetButtonDown ("Fire1")) {
                 Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
 
                 Instantiate(testTowerPrefab[0], Input.mousePosition, transform.rotation);
                 Debug.Log ("Instantiated Tower1");
 
                 towerBools[0] = false;
 
             }
         }
     }
 
     void OnGUI()
     {
         GUI.Box (new Rect (Screen.width - 300, 0, Screen.width / 4, Screen.height - 2), "", boxStyle);
         if (GUI.Button (new Rect (Screen.width - 300 / 2, 50, 50, 50), "Tow Pic", buttonStyle)) {
             towerBools[0] = true;
         }
     }