Unable to use costume hybrid on Space Pirate Costume

[Davodile]

I recently bought the Devil Horn Christmas costume headpiece for assassin not because I like the looks of it, I was only after its OP stats.
I was hoping I could use my Costume Hybrid Tablet to synthesize my Devil Horn headpiece with my awesome Space Pirate hat.
But neither of both items are synthesizable.
It seems that all of my other costumes are synthesizable except the Space Pirate set. Is this meant to be?