Swift错误：在展开Optional值时意外发现nil

func fontForDisplay(atIndexPath indexPath : NSIndexPath) -> UIFont? {
    if indexPath.section == 0 {
        let familyName = familyNames[indexPath.row]
        let fontName = UIFont.fontNamesForFamilyName(familyName).first as String!
        return UIFont(name: fontName, size: cellPointSize)
    }else {
        return nil
    }
}
func fontForDisplay(atIndexPath indexPath : NSI