Could not cast value of type 'NSTaggedPointerString' to 'NSNumber'

The reason of the error is jsonDict["totalfup"] is a String (NSTaggedPointerString is a subclass of NSString) , so you should convert String to Double.

Please make sure, catch exception and check type before force-unwrap !

totalData = (jsonDict["totalfup"] as! NSString).doubleValue

For safety, using if let:

// check dict["totalfup"] is a String?if let totalfup = (dict["totalfup"] as? NSString)?.doubleValue {  // totalfup is a Double here }else {  // dict["totalfup"] isn't a String  // you can try to 'as? Double' here}

I think this could help you

totalData = Double(jsonDict["totalfup"] as! String)!

The failure reason is that the JSON returns String values, not numbers.

If the returned JSON data contains only these two key value pairs declare the type as [String:String] that avoids the type casting.

In any case you have to put the code to update the variables into the "good" branch of the do - catch expression.

struct Usage {    var totalData = 0.0    var remainingTotalData = 0.0    init(jsonData: NSData) { // Swift 3: Data        do {            let jsonDict = try NSJSONSerialization.JSONObjectWithData(jsonData, options: []) as! [String: String]                            // Swift 3: let jsonDict = try NSJSONSerialization.jsonObject(with: jsonData) as! [String: String]            totalData = Double(jsonDict["totalfup"]!)            remainingTotalData = Double(jsonDict["totalrem"]!)        } catch {            print("Error occurred parsing data: \(error)")        }    }}