UserDefaults save String -> Attempt to set a non-property-list object (Function) as an NSUserDefaults/CFPreferences

i get the error message above when retrieving UserDefaults. I saved the value as a String. TimeInterval is converted to String. Where is the problem?

Code for saving the string:

let timeResponse:TimeInterval = NSDate.timeIntervalSinceReferenceDate - start

let JSONTransferTime = TimeInterval.toReadableString(timeResponse) //converting to String

UserDefaults.standard.set(JSONTransferTime, forKey:"JSONTranferTime")

Code for retrieving the string:

let JSONTransferTime: String = UserDefaults.standard.string(forKey: "JSONTranferTime")! as String

Utils.displayAlert(title: "JSON download time", message: JSONTransferTime)

Exact error message:

[User Defaults] Attempt to set a non-property-list object (Function) as an NSUserDefaults/CFPreferences value for key JSONTranferTime

Code for toReadableString:

extension TimeInterval {

func toReadableString() -> String {

    // Nanoseconds
    let ns = Int((self.truncatingRemainder(dividingBy: 1)) * 1000000000) % 1000
    // Microseconds
    let us = Int((self.truncatingRemainder(dividingBy: 1)) * 1000000) % 1000
    // Milliseconds
    let ms = Int((self.truncatingRemainder(dividingBy: 1)) * 1000)
    // Seconds
    let s = Int(self) % 60
    // Minutes
    let mn = (Int(self) / 60) % 60
    // Hours
    let hr = (Int(self) / 3600)

    var readableStr = ""
    if hr != 0 {
        readableStr += String(format: "%0.2dhr ", hr)
    }
    if mn != 0 {
        readableStr += String(format: "%0.2dmn ", mn)
    }
    if s != 0 {
        readableStr += String(format: "%0.2ds ", s)
    }
    if ms != 0 {
        readableStr += String(format: "%0.3dms ", ms)
    }
    if us != 0 {
        readableStr += String(format: "%0.3dus ", us)
    }
    if ns != 0 {
        readableStr += String(format: "%0.3dns", ns)
    }

    return readableStr
}

}

JSONTransferTime-variable is filled:

toReadableString() is an instance method, you have to call it on timeResponse rather than on the type which returns the function the compiler is complaining about by the way.

let jsonTransferTime = timeResponse.toReadableString()

Please conform to the naming convention that variable names start with a lowercase letter and don't annotate types the compiler can infer.