Подскажите, в чем ошибка, валидатор не принимает

package com.javarush.task.task09.task0923;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;

/*
Гласные и согласные
*/

public class Solution {
    public static String vowels = "аяуюиыэеоё";

    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        char[] arr = br.readLine().replaceAll(" ", "").toCharArray();
        StringBuilder a = new StringBuilder(""), b = new StringBuilder("");

        isVowel(arr, a, b);
        System.out.println(b);
        System.out.print(a);

    }

    public static void isVowel(char[] c, StringBuilder a, StringBuilder b) {
        for(int i = 0; i < c.length; ++i) {
            c[i] = Character.toLowerCase(c[i]);
            if(vowels.indexOf(c[i]) > -1)
                b.append(c[i] + " ");
            else if (c[i] != ' ')
                a.append(c[i] + " ");
        }
    }
}