Why am I getting a NullReferenceException ?

I am raycasting on an enemy prefab I have. The enemy has a rigidbody on it. the prefab is tagged “Ground Enemy”. I am getting a NullReferenceException from the raycast.
here is my code:

var hit : RaycastHit;
		if (Physics.Raycast(rayorigin.position, turretBall.forward, hit)) {
    		if (hit.rigidbody.tag == "Ground Enemy") {
    			rigidbody.SendMessage("TakeDamage", myDamageAmount, SendMessageOptions.DontRequireReceiver);

I am new to using raycasting. To my understanding though I shouldn’t be getting any feedback unless the ray is hitting an object with a rigidbody with a tag == “Ground Enemy” and if that is the case I don’t understand why it is getting a null reference…

On line 4, you are using 'rigidbody.SendMessage(). This will attempt to use a rigidbody of the game object the script above is attached to. I believe you want:

 hit.rigidbody.SendMessage("TakeDamage", myDamageAmount, SendMessageOptions.DontRequireReceiver);

Note you are probably safer for what you are doing here to use ‘hit.collider.tag’ and ‘hit.collider.SendMessage()’. Not all game object hit have a rigidbody, but all object hit must have a collider.