Help with C# XNA/Monogame

I need the character to be able to shoot fireballs only every 12 seconds. But I can still shoot before 12 seconds and I don't want that. So when I shoot a fireball I try to record it as the current GameTime.ElapsedGameTime value but it's not working.

here is the code

// Create a readonly variable which specifies how often the user can shoot. 

 private static readonly TimeSpan ShootInterval = TimeSpan.FromSeconds(12);

// Keep track of when the user last shot. Or null if they have never shot in the current session.

        private TimeSpan? LastFireballShot;


private void UpdateFireball(GameTime theGameTime, KeyboardState aCurrentKeyboardState)
        {
            foreach (Fireball aFireball in mFireballs)
            {
                aFireball.Update(theGameTime);
            }

            if (aCurrentKeyboardState.IsKeyDown(Keys.RightControl) == true && mPreviousKeyboardState.IsKeyDown(Keys.RightControl) == false)
                  

            {
//the user is trying to shoot

                if (LastFireballShot == null || theGameTime.ElapsedGameTime - (TimeSpan)LastFireballShot >= ShootInterval)
                {

// Allow the user to shoot because he either has not shot before or it's been 1 second since the last shot.

                    ShootFireball();
                    
                }

            }