where is the bug?

fail.swift

    func testOMGWTF() {
        var err: NSError?
        let plist = [
            "a": "c",
            // note that 1 is a String
            "b": "1"
        ]
        let data = NSPropertyListSerialization.dataWithPropertyList(
            plist,
            format: NSPropertyListFormat.BinaryFormat_v1_0,
            options: 0,
            error: nil)

        if let properties = NSPropertyListSerialization.propertyListWithData(
            data,
            options: 0,
            format: nil,
            error: &err) as? NSDictionary {
                println("heyo")
        } else {
            XCTFail(err!.description)
        }

pass.swift

    func testOMGWTF() {
        var err: NSError?
        let plist = [
            "a": "c",
            // making this an Int is the difference
            "b": 1
        ]
        let data = NSPropertyListSerialization.dataWithPropertyList(
            plist,
            format: NSPropertyListFormat.BinaryFormat_v1_0,
            options: 0,
            error: nil)

        if let properties = NSPropertyListSerialization.propertyListWithData(
            data,
            options: 0,
            format: nil,
            error: &err) as? NSDictionary {
                println("heyo")
        } else {
            XCTFail(err!.description)
        }