Error when running strokeChart

[beamercola]

Assertion failed: (CGFloatIsValid(x) && CGFloatIsValid(y)), function void CGPathMoveToPoint(CGMutablePathRef, const CGAffineTransform *, CGFloat, CGFloat), file Paths/CGPath.cc, line 254.

It's happening on progressline.moveToPoint(CGPointMake(x, y))

My code is pretty simplistic:

            var weekData:NSDictionary = JSON["week"] as! NSDictionary
            // Example: { "02/15": 7 }

            self.chartView?.xLabels = weekData.allKeys
            var data = PNLineChartData()
            data.color = UIColor(red: 77.0 / 255.0, green: 186.0 / 255.0, blue: 122.0 / 255.0, alpha: 1.0)
            data.itemCount = weekData.allKeys.count
            data.getData = ({(index: Int) -> PNLineChartDataItem in
                var yValue:CGFloat = weekData.allValues[index] as! CGFloat
                var item = PNLineChartDataItem(y: yValue)
                return item
            })
            self.chartView?.chartData = [data]
            self.chartView?.strokeChart()