Need help with reloading

void Reload()
numberofBulletsInGun += shotsFired;
GunsAmmo.Instance.revolverAmmo -= shotsFired;
shotsFired = 0f;

numberOfBulletsInGun - how much bullets are left in the gun,

shotsFired - Counts how many times the player fired the gun,
GunsAmmo.Instance.revolverAmmo - Ammo left for the gun,

The problem I’m having is that when there is less than 9 bullet in the GunsAmmo.Instance.revolverAmmo and the numberOfBulletsInGun is empty when I press R to reload
GunsAmmo.Instance.revolverAmmo goes into the negative while numberOfBulletsInGun goes to 9. This only doesn’t work if numberOfBulletsInGun is 0.

Also sorry for bad grammer, English is my second language.