projectile models not working

Hello. When I am creating a ranged item, and set the projectile model and texture to a custom one, It doesn't compile properly. when I look into the item, the "item representing the texture of the projectile" slot is empry and when I add something in, it still doesn't work. however, when I only fill the "item representing the texture of the projectile" slot, It works. I am not able to make projectiles with custom models because of this.