An invisible barrier of magical force appears and protects you. Until the start of your next turn, you change your armour class so it cannot be lower than 20 potentially changing the triggering attack into a miss. You also take no damage from the magic missile spell for the duration.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the armour class you gain from this spell increases by 1 for each slot level above 1st.