Cannot invoke '+=' with an argument list of (Double, @lvalue Double)

Question

I'm running into a weird error by Xcode:

Cannot invoke '+=' with an argument list of (Double, @lvalue Double)

Given the following code using iOS SDK 8.1 and Xcode 6.1.1. (You can simply put this into a Playground and test it.)

class Mark {
    var description: String
    var mark: Double
    var weight: Double

    init(description: String, mark: Double, weight: Double) {
        self.description = description
        self.mark = mark
        self.weight = weight
    }
}

// in some other class
var marks = [Mark(description: "", mark: 5, weight: 1), Mark(description: "", mark: 4, weight: 0.5)] // sample data

var totalMarks: Double {
    let total = 0.0
    for mark in marks {
        total += mark.mark // Cannot invoke '+=' with an argument list of (Double, @lvalue Double)
    }
    return total
}

Xcode says total is a Double, mark is a Mark and I assume mark.mark must be a Double as defined.

Am I missing to import a module? I only import Foundation at the moment, which I guess should be enough.