Problem with Swift GuidedTour enum with protocol challenge

SwiftGuidedTourEnum.swift

protocol ExampleProtocol {
     var simpleDescription: String { get }
     mutating func adjust()
}

class SimpleClass: ExampleProtocol {
     var simpleDescription: String = "A very simple class."
     var anotherProperty: Int = 69105
     func adjust() {
          simpleDescription += "  Now 100% adjusted."
     }
}
var a = SimpleClass()
a.adjust()
let aDescription = a.simpleDescription

struct SimpleStructure: ExampleProtocol {
     var simpleDescription: String = "A simple structure"
     mutating func adjust() {
          simpleDescription += " (adjusted)"
     }
}
var b = SimpleStructure()
b.adjust()
let bDescription = b.simpleDescription

// Can't get this to work.
enum SimpleEnum: ExampleProtocol {
    case A, B
    var simpleDescription: String = "A very simple enum."
    mutating func adjust() {
        simpleDescription += "  Now 110% adjusted."
    }
    
}