Negative Joystick Values Don't Control Mecanum Wheels

[MichaelB]

In my team's mecanum drive system, negative joystick values don't control the wheels, while positive ones do. Can anyone help with this? I've tried looking in the WPILib code, but can't find anything that would do it.

Relevant code:
DriveWithJoystick

protected void execute() {
        double x = oi.getRightStick().getX();
        double y = oi.getRightStick().getY();
        double twist = ((oi.getLeftStick().getX())-0.5)*2; //This is because all the way to the left is being interpreted as 0, while all the way to the right is 1.
        driveTrain.driveWithJoystick(x, y, twist);
        lcd.println(DriverStationLCD.Line.kUser2, 1, "Y: " + ((int)(y*10))/10.0);
        lcd.println(DriverStationLCD.Line.kUser3, 1, "Y: " + ((int)(y*10))/10.0);
        lcd.println(DriverStationLCD.Line.kUser4, 1, "Twist: " + ((int)(twist*10))/10.0);
        lcd.updateLCD();
    }

DriveTrain

    public void driveWithJoystick(double x, double y, double twist) {
        //x = -((x - Robot3573.initialX) / (1 - Robot3573.initialX));
        //y = (y - Robot3573.initialY) / (1 - Robot3573.initialY);
        //twist = (twist - Robot3573.initialTwist) / (1 - Robot3573.initialTwist);
        x = x > RobotMap.joystickTolerance ? x : 0;
        y = y > RobotMap.joystickTolerance ? y : 0;
        twist = twist > RobotMap.joystickTolerance ? twist : 0;
        driveTrain.mecanumDrive_Cartesian(x,y,twist, 0);
        System.out.println("X: " + x + "Y: " + y + "Twist: " + twist);
        //driveTrain.mecanumDrive_Polar(x,y,twist);
    }