How can I change a variable for a newly instantiated object?

I have a prefab called "Shell". It has this script attached:

 public class ShellExplosion : MonoBehaviour 
 { public int sentFromPlayer = 0; } //this is the variable I need to change

I also have a "Player" that has this other script:

 public class PlayerScript : MonoBehaviour
 {
 public Rigidbody m_Shell; // I've put the "shell" prefab here 
 private void Fire() 
 {  
  Rigidbody shellInstance = Instantiate(m_Shell, thisPosition, thisRotation) as Rigidbody;
  shellInstance.gameObject.GetComponent<ShellExplosion>().sentFromPlayer=1;
 }
 }

I get the error 'ShellExplosion' does not contain a definition for 'sentFromPlayer'.

I also tried instantiating as a game object (instead of rigid body), but I get the same error. I'm sure the names match. Any help would be appreciated. Thank you