Goblin Negotiation dealt 10 to my opponent

During Quick Battle - Standard, I cast a fully charged Goblin Negotiation, dealing 10 damage to an opposing Calix, Guided by Fate (4/4). I control an Emberheart Challenger and a Searslicer Goblin, with no other supports or effects. Negotiation's Trample like effect dealt 10 damage to my opponent, when it should have dealt 6.