Nil, incompatible cgaffinetransform

Question

I'm getting a Nil is not compatible with expected argument type 'unsafePointer' error for this

let centerX = bounds.midX
    let centerY = bounds.midY

    let outerRadius = outerRadiusInRect(rect: bounds)
    let innerRadius = outerRadius - GNTickerButton.kOutterInnerRingSpacing

    let path = CGMutablePath()
    let padding = 8 as CGFloat

    CGPathAddArc(path, nil, centerX, centerY, GNTickerButton.kTearDropRadius,  CGFloat(2*M_PI),  CGFloat(M_PI), false)