When getting the list of sets through the info call promotion is named promo.
Everywhere else it is named promotion.

URL: https://omgvamp-hearthstone-v1.p.mashape.com/info

Response:

{
“patch”: “2.8.0.9554”,
“classes”: [
“Druid”,
“Hunter”,
“Mage”,
“Paladin”,
“Priest”,
“Rogue”,
“Shaman”,
“Warlock”,
“Warrior”,
“Dream”
],
“sets”: [
“Basic”,
“Classic”,
“Credits”,
“Naxxramas”,
“Debug”,
“Goblins vs Gnomes”,
“Missions”,
“Promo”,
“Reward”,
“System”,
“Blackrock Mountain”,
“Hero Skins”,
“Tavern Brawl”
],
“types”: [
“Hero”,
“Minion”,
“Spell”,
“Enchantment”,
“Weapon”,
“Hero Power”
],
“factions”: [
“Horde”,
“Alliance”,
“Neutral”
],
“qualities”: [
“Free”,
“Common”,
“Rare”,
“Epic”,
“Legendary”
],
“races”: [
“Demon”,
“Dragon”,
“Mech”,
“Murloc”,
“Beast”,
“Pirate”,
“Totem”
],
“locales”: [
“deDE”,
“enGB”,
“enUS”,
“esES”,
“esMX”,
“frFR”,
“itIT”,
“koKR”,
“plPL”,
“ptBR”,
“ptPT”,
“ruRU”,
“zhCN”,
“zhTW”
]
}