Can't get SequenceType to work in Swift Beta 6

Question

In beta 5 this code works perfectly, but it doesn't compile in Beta 6. Any ideas why?

struct GenericGenerator<T>: GeneratorType {
    var items: [T]
    mutating func next() -> T? {
        return items.isEmpty ? .None : items.removeAtIndex(0)
    }
}

class ExerciseContainer : SequenceType {

    var exercises: [Exercise] = []

    func generate() -> GenericGenerator<Exercise> {
        return GenericGenerator(items: self.exercises)
    }
}