Input.mousePosition changing to (0,0) after GetMouseButtonUp

I'm trying to make my Player be able to throw a bomb with a vector which is directly determined by the mouse positions at GetMouseButtonDown and GetMouseButtonUp. The code is as follows (all in 2D, btw):

 #pragma strict
 
 public var newBomb:Rigidbody2D;
 public var ThrowFactor:float = 1;
 public var destructTime:float = 3;
 
 function Start () {
 
 }
 
 function Update () 
 {
     if(Input.GetMouseButtonDown(0))
     {
         var start_pos:Vector2 = Input.mousePosition;
         Debug.Log("start1 = "+start_pos);
     }
     if (Input.GetMouseButtonUp(0))
     {
         var end_pos:Vector2 = Input.mousePosition;
         var throwDirection:Vector2 = ThrowFactor*(end_pos-start_pos);
         Debug.Log("start2 = "+start_pos);
         Debug.Log("end = "+end_pos);
         Debug.Log("direction = "+throwDirection);
         var new_pos : Vector3 = transform.position+throwDirection.normalized;
         var new_rot : Quaternion = Quaternion(0,0,0,0);
         var newBombInstance : Rigidbody2D;
         newBombInstance = Instantiate(newBomb, new_pos, new_rot);
         newBombInstance.gravityScale =10;
         newBombInstance.velocity = throwDirection;
     }
 }

For some reason, the debug.log always shows (start2 = 0.0,0.0) even when start1 is not. Any ideas?