Initializing Swift Optional in Class Method

Question

New at Swift and playing around with optionals and how they work. Is there some reason you can't initialize an optional in a struct method? If you change 'A' to be a class then this all works. Using Xcode 6.3.1.

struct A {
    var myString: String?

    init () {
        // do something
    }

    func getData () {
        // This next line doesn't compile correctly
        //self.myString = "This is a test"
    }    
}

var a = A()
a.myString = "Test" // This line works