Swift：EXC_BAD_ACCESS错误访问字典上的键

Question

游乐场，Xcode 9.4.1

let openersToClosers: [Character: Character] = ["(": ")","[": "]","{": "}"]
print("openersToClosers.keys : \(openersToClosers.keys)")

let keys = openersToClosers.keys
print("keys : \(keys)")

我遇到一个错误：