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;

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

I was close to going crazy over this… after some extended search, I found the issue:
My player script was inside a different namespace.