Has Harry spelled his underpants? (Spoilers: Turn Coat)

Ok, hear me out...

When Harry performs the Sanctum invocation in Turn Coat, he does so naked:
Turn Coat

Why completely naked? We know about the duster and the jewellery. Maybe, at a stretch, his shirts catch a lot of residual magic flowing through his arms (in on the left, out on the right, IIRC). But really, he couldn't keep his undies on? Only one explanation: he has protective spells on there to protect him from a sneaky nut shot in combat!

Now maybe you are thinking "he goes commando", but we know he doesn't at least usually:
Something Borrowed

Or maybe his diminutive housekeepers use a magic detergent?