Instantiate in C#?

I'm trying to create a new gameobject and then add a force to it's rigidbody. I followed the example in the reference but it somehow doesnt work. I always get the error: "Cannot cast from source type to destination type." (The instantiate line)

void Shoot()
{

Rigidbody Bullet = (Rigidbody) Instantiate (Munitionstyp,Hilfsvektor,Austrittspunkt.rotation);
Bullet.AddForce(Austrittspunkt.forward * Bulletspeed);

}

Im not sure, im usually using javascript, and maybe im totally wrong, but this might work?

void Shoot()
{
Rigidbody Bullet;
Bullet = Instantiate (Munitionstyp,Hilfsvektor,Austrittspunkt.rotation);
Bullet.AddForce(Austrittspunkt.forward * Bulletspeed);
}