import Foundation enum E: Codable { case a(Int???) } let decoder = JSONDecoder() let json = """ [ { "a" : { } }, { "a" : { "_0" : null } }, { "a" : { "_0" : null } }, { "a" : { "_0" : 1 } } ] """.data(using: .utf8)! let value = try decoder.decode([E].self, from: json) dump(value)