почему выдает Exception?

package com.javarush.task.task08.task0817; import java.util.HashMap; import java.util.Map; /* Нам повторы не нужны */ public class Solution { public static Map<String, String> createMap() { Map<String, String> map = new HashMap<>(); map.put("sdf", "rere"); map.put("zcv", "erer"); map.put("gdb", "rere"); map.put("fvs", "dffd"); map.put("asfva", "zdzd"); map.put("sd", "fdfd"); map.put("asdv", "zdzd"); map.put("zc", "gfsge"); map.put("ZVD", "fvsd"); map.put("d", "d"); return map; } public static void removeTheFirstNameDuplicates(Map<String, String> map) { for (Map.Entry<String, String> pairMap : map.entrySet()) { Map<String, String> copyOfMap = new HashMap<>(map); copyOfMap.remove(pairMap.getKey()); for (Map.Entry<String, String> pairCopyOfMap : copyOfMap.entrySet()) { if (pairMap.getValue().equals(pairCopyOfMap.getValue())) { removeItemFromMapByValue(map, pairMap.getValue()); } } } } public static void removeItemFromMapByValue(Map<String, String> map, String value) { Map<String, String> copy = new HashMap<>(map); for (Map.Entry<String, String> pair : copy.entrySet()) { if (pair.getValue().equals(value)) { map.remove(pair.getKey()); } } } public static void main(String[] args) { Map<String, String> map = createMap(); removeTheFirstNameDuplicates(map); for (Map.Entry<String, String> pair : map.entrySet()) { System.out.println(pair); } } }