Test invalid number format input using junit

I am trying learn Junit and i stumble up on this. at the moment i am trying to testing this part of my code

        while (true) {
        System.out.println("Enter Marks for Assingment B:");
        try {
            MarksB = Double.parseDouble(data.next());
            if (MarksB<0||MarksB>50){
            System.out.println("Please enter a number between 0-50");
        }
            else{
            break;
            }
        } catch (NumberFormatException e) {
            System.out.println("Please enter a valid number");
            System.out.println(e);
        }
    }

Trying to Test this part of using Junit, but i can not figure out way to insert a string input MarksB varaible (double), This is the code iv written so far

    @Test (expected = NumberFormatException.class)
public void testInvalidInput(){
    double sum = Testing.add2Marks("string", 12);
}

Thanks in advance