Hi, I'm new to Swift and app development so please do not hate me. Here's my code:     @IBAction func equalsTap(_ sender: Any) {         if(validInput()){             let checkedWorkingForPercent = workings.replacingOccurrences(of: "%", with: "*0.01")             let expression = NSExpression(format: checkedWorkingForPercent)             let result = expression.expressionValue(with: nil, context: nil) as! Double             let resultString = formatResult(result: result)             calculatorResults.text = resultString                      }         else {         }     } And I get an error Thread 1: signal SIGABRT at the line let result = expression.expression... Could you help me, please?