Swift beta 6 – 混乱的链接器错误消息

Undefined symbol for architecture x86_64:
__TFSs26_forceBridgeFromObjectiveCU__FTPSs9AnyObject_MQ__Q_",referenced from:
 __TFC8RayTrace14RayTracingPlot15drawFocalPointfS0_FT_T_ in RayTracingPlot.o
ld: symbol(s) not found for architecture x86_64

这里是有问题的代码：

private func drawFocalPoint() {
    var attributes = Dictionary<String,AnyObject>()

    let FString: String = "F"
    let distance: CGFloat = focalDistance

    let centerX = CGRectGetMidX(bounds)
    let centerY = CGRectGetMidY(bounds)

    let circleRadius: CGFloat = 4.0
    let focalPointFrame = CGRectMake(0,circleRadius * 2.0,circleRadius * 2.0)
    var path = UIBezierPath(ovalInRect: focalPointFrame)
    let color = UIColor.blackColor()

    let currentContext = UIGraphicsGetCurrentContext()
    CGContextSaveGState(currentContext)
    let shadowColor = UIColor(white:0,alpha:0.75).CGColor
    CGContextSetShadowWithColor(currentContext,CGSizeMake(0,4),CGFloat(8),shadowColor)

    // Image F
    var imageFPath = UIBezierPath(CGPath: path.CGPath)
    let imageFTransform = CGAffineTransformMakeTranslation((centerX - distance - circleRadius),(centerY - circleRadius))
    imageFPath.applyTransform(imageFTransform)
    color.set()
    imageFPath.fill()
    FString.drawAtPoint(CGPointMake(centerX - distance - circleRadius,centerY + 5),withAttributes:attributes)

    CGContextSetShadowWithColor(currentContext,CGSizeMake(0.0,0.0),CGFloat(0.0),nil) // Clear shadow
    CGContextRestoreGState(currentContext)
}

我会欣赏一个提示，在这个代码中寻找错误，所以我可以解决它。谢谢。