Extension of Progress Bar causing warning of not key value-coding compilant

Question

I set up an extension to change the height of a progress bar:

extension UIProgressView {
    @IBInspectable var barHeight : CGFloat {
        get {
            return transform.d * 2.0
        }
        set {
            // 2.0 Refers to the default height of 2
            let heightScale = newValue / 2.0
            let c = center
            transform = CGAffineTransformMakeScale(1.0, heightScale)
            center = c
        }
    }
}

and then set the height using this of the progress bars in viewDidLoad()

EncounteredBar.barHeight = 10

And then eventually got the warning:

2016-12-19 22:04:02.249 CYOA[1169:17032] Failed to set (barHeight) user defined inspected property on (UIProgressView): [<UIProgressView 0x7f834bd520d0> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key barHeight.

The annoying part is even when I remove the extension and the setting of barHeight from the code, this warning still appears. I would like to know how to get rid of this warning.