Nil is incompatible with return type 'MKOverlayRenderer'

Question

I am getting this error

"Nil is incompatible with return type 'MKOverlayRenderer'".

Here is my code:

func mapView(mapView: MKMapView, rendererForOverlay overlay: MKOverlay) -> MKOverlayRenderer {
    if overlay is MKCircle {
      let circleRenderer = MKCircleRenderer(overlay: overlay)
      circleRenderer.lineWidth = 1.0
      circleRenderer.strokeColor = UIColor.purpleColor()
      circleRenderer.fillColor = UIColor.purpleColor().colorWithAlphaComponent(0.4)
      return circleRenderer
    }
      return nil
  }

why is it occurring?

Answer 1

return MKPolylineRenderer() instead of returning nil.

Answer 2

This seems to be answered here:

Swift 2 MKMapViewDelegate rendererForOverlay optionality

You are just not allowed to return nil. Your overlay must have a renderer. Replace "return nil" with an assertion.