Instantiating an object and giving a velocity causes NullReferenceException

I'm trying to set the velocity of an instantiated object using the following code. However, line 22 is causing a NullReferenceException. I understand this must mean that the probe3DClone variable must be returning null, but I'm confused as to why (as far as I can tell the same code is used in the Unity documentation). How can I make this work?

Code (csharp):

1.  
2.     void Update ()
3.     {
4.  
5.         //When Fire1 is pressed, create a probe at launcher location
6.         if (Input.GetButton("Fire1") && Time.time > nextFire)
7.         {
8.  
9.             //Get click location of mouse
10.             clickLocation = Input.mousePosition;
11.             clickLocation = Camera.main.ScreenToWorldPoint(clickLocation);
12.  
13.             nextFire = Time.time + fireRate;
14.  
15.             //Variable to reference instantiated probe2D
16.             Rigidbody probe3DClone;
17.             //Creates probe3D at probeSpawn
18.             probe3DClone = Instantiate (probe3D, probe3DSpawn.position, probe3DSpawn.rotation) as Rigidbody;
19.             //Set probe3D velocity
20.             probe3DClone.velocity = new Vector3(clickLocation.x, ySpeed, zSpeed);
21.         }
22.  
23.     }
24.  

 

Figured it out. For some reason I couldn't store the instantiated object as a Rigidbody, but I could as a GameObject. Used the following:

Code (csharp):

1.  
2.             //Variable to reference instantiated probe2D
3.             GameObject probe3DClone;
4.             //Creates probe3D at probeSpawn
5.             probe3DClone = Instantiate (probe3D, probe3DSpawn.position, probe3DSpawn.rotation) as GameObject;
6.  
7.             //Set probe3D velocity
8.             probe3DClone.GetComponent<Rigidbody>().velocity = new Vector3(clickLocation.x, ySpeed, zSpeed);
9.