Null Reference Exception when trying to active script

Hello,

When my marble collide with the gravity pickup, then the marble should activate the activation script on the gravity pickup. I am using the following code on my marble:

 function OnTriggerEnter (other : Collider)
 {
     if (other.tag == "gravity")
     {
         other.GetComponent("Gravity_defier").Activate();
     }

}

And the gravity object uses the following code:

 function Activate()
 {
     Physics.gravity = Vector3(0, -gravityAmmount, 0);
 }

However, when my marble is triggers the object, I get Null Reference Exception: Object reference not set to an instance of an object. The marble has a rigidbody, and a spere collider, and the gravity pickup has a capsule collider set to trigger.

How should I fix that?