nextLine() issue -- beginner

[dark cloud]

import java.util.Scanner;
 public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int sum = 0;
        while (true) {
            System.out.print("Please enter a number: ");
            int num = scanner.nextInt();
            if (num % 2 == 0) {
                sum += num;
                System.out.println("sum = " + sum);
                continue;
            }
            scanner.nextLine(); // this line
            System.out.print("Do you want to continue?(Y/N): ");
            String response = scanner.nextLine();
            if (response.toLowerCase().charAt(0) == 'n') {
                break;
            }
        }
        System.out.println("Total sum = " + sum);
    }
}

Above can run; my question is, if I delete the line with comment: scanner.nextLine()
Why it gives error: Exception in thread "main" java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0

[chrome walrus]

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[shy holly]

https://stackoverflow.com/q/13102045/10871900