Ios 向Swift中的属性文本添加属性

我尝试向文本标签添加属性：

let paragraphStyle = NSMutableParagraphStyle()
paragraphStyle.alignment = NSTextAlignment.Center

let para = NSMutableAttributedString()
let font:UIFont? = UIFont(name: "DIN Alternate", size: 18.0)
let font2:UIFont? = UIFont(name: "Arial", size: 12.0)
let labelColor = UIColor.whiteColor()
let text="some text"

//A, error appears here
let myAttributes = [NSParagraphStyleAttributeName:paragraphStyle,
                    NSFontAttributeName:font,
                    NSForegroundColorAttributeName:labelColor]

let attrString = NSAttributedString(
    string: csvObject.provideDetailsAsString(poi)!,
    attributes: myAttributes)
para.appendAttributedString(attrString)     
detailsText.attributedText = para

但是Swift编译器在//A行中出现错误，并表示：

'_' is not convertible to 'String'

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我正在使用XCode 6.4和Swift 1.2找到它，添加一个！在字体之后：NSFontAttributeName:font

字体是可选的