Creating a PDF image from CALayer

I'm attempting to create a PDF image from a CALayer. The image gets saved to disk, however I can't open it in Preview or Photoshop. So presumably my code isn't creating it correctly.

        let layer = CALayer()

        let line = CALayer()
        line.backgroundColor = NSColor.blackColor().CGColor
        line.frame = CGRectMake(0, 10, 5, 10)
        layer.addSublayer(line)

        let line2 = CALayer()
        line2.backgroundColor = NSColor.blackColor().CGColor
        line2.frame = CGRectMake(0, 10, 5, 10)
        layer.addSublayer(line2)

        let pdfData: NSMutableData = NSMutableData()
        let pdfConsumer = CGDataConsumerCreateWithCFData(pdfData as CFMutableDataRef)
        var mediaBox: CGRect = CGRectMake(0, 0, 20, 20)
        let pdfContext = CGPDFContextCreate(pdfConsumer, &mediaBox, nil)
        CGContextBeginPage(pdfContext, &mediaBox)
        layer.renderInContext(pdfContext!)
        CGContextEndPage(pdfContext)

        let image = NSImage(data: pdfData)
        do {
            try pdfData.writeToURL(NSURL.fileURLWithPath(imgPath), options: NSDataWritingOptions.AtomicWrite)
        } catch {
            print(error)
        }