During my previous turn I had cast onto the battlefield Aven Wind Mage, which has the ability:
Whenever you cast an instant or sorcery spell, Aven Wind Mage gets +1/+1 until end of turn.
On their next turn, my opponent activated the ability of Siege-Gang Commander to deal two damage to my Wind Mage, attempting to destroy it:
{1}{R}, Sacrifice a Goblin: Siege-Gang Commander deals 2 damage to any target.
I responded to this with Dramatic Reversal:
Untap all nonland permanents you control.
even though I knew that all my creatures were already untapped. My opponent claimed that this was illegal as Aven Wind Mage has summoning sickness and does not get to trigger its ability, keeping it alive.
Who is correct in this situation - can I legally play Dramatic Reversal although there are no tapped creatures to untap and as a result give Aven Wind Mage the needed +1/+1 until the end of turn to keep it alive?