Camera Switcher script

I am having a problem with my script, I am trying to switch to a Rear_cam when the V key is held down and when it is released have it switch back to Main_Camera. When I attempt to use it it gives me this error message:

NullReferenceException: Object reference not set to an instance of an object
Boo.Lang.Runtime.RuntimeServices.GetDispatcher (System.Object target, System.String cacheKeyName, System.Type cacheKeyTypes, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory)
Boo.Lang.Runtime.RuntimeServices.GetDispatcher (System.Object target, System.Object args, System.String cacheKeyName, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory)
Boo.Lang.Runtime.RuntimeServices.SetProperty (System.Object target, System.String name, System.Object value)
Camswitcher.Update () (at Assets/Scripts/Camswitcher.js:7)

Here is the script I made:

public var Rear_cam :Camera;
public var Main_Camera : Camera;

  function Update(){
  if (Input.GetKeyDown ("v"))
{
  Rear_cam.Camera.enabled = true;
  Main_Camera.Camera.enabled = false;

}

  if (Input.GetKeyUp ("v"))
{
  Rear_cam.Camera.enabled = false;
  Main_Camera.Camera.enabled = true;

}
}

Can anyone help me out here? (I’m kind of new to unity)

Capitalization is important in programming. Camera is a class, camera is a member property. It’s recommended that you follow Unity’s standards of CamelCase for functions and classes, and camelCase for members, to prevent confusion.

Have you assigned a value to your cameras in the inspector?