mapAB2 errors

http://codingbat.com/prob/p115011

For above challenge i wrote as below

public Map<String, String> mapAB2(Map<String, String> map) {
  if( map.get("a").equals(map.get("b")) ){
    map.remove("a");
    map.remove("b");
  }
  return map;
}

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Expected      Run            
mapAB2({"a": "aaa", "b": "aaa", "c": "cake"}) → {"c": "cake"}      {"c": "cake"}      OK      
mapAB2({"a": "aaa", "b": "bbb"}) → {"a": "aaa", "b": "bbb"}      {"a": "aaa", "b": "bbb"}      OK      
mapAB2({"a": "aaa", "b": "bbb", "c": "aaa"}) → {"a": "aaa", "b": "bbb", "c": "aaa"}      {"a": "aaa", "b": "bbb", "c": "aaa"}      OK      
mapAB2({"a": "aaa"}) → {"a": "aaa"}      {"a": "aaa"}      OK      
mapAB2({"b": "bbb"}) → {"b": "bbb"}      NullPointerException (line:2)      X      
mapAB2({"a": "", "b": "", "c": "ccc"}) → {"c": "ccc"}      {"c": "ccc"}      OK      
mapAB2({}) → {}      NullPointerException (line:2)      X      
mapAB2({"a": "a", "b": "b", "z": "zebra"}) → {"a": "a", "b": "b", "z": "zebra"}      {"a": "a", "b": "b", "z": "zebra"}      OK      
other tests
X      

i am failing one edge case. how to fix it?
please advise on any modifications, improvement to the code

public Map<String, String> mapAB2(Map<String, String> map) {
  if( map.get("a")!=null && map.get("b")!=null && map.get("a").equals(map.get("b")) ){
    map.remove("a");
    map.remove("b");
  }
  return map;
}

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above passes all tests but challenge description did not mention a and b values cannot be null or can be null etc?

Modify and return the given map as follows: if the keys "a" and "b" are both in the map and have equal values, remove them both.

mapAB2({"a": "aaa", "b": "aaa", "c": "cake"}) → {"c": "cake"}
mapAB2({"a": "aaa", "b": "bbb"}) → {"a": "aaa", "b": "bbb"}
mapAB2({"a": "aaa", "b": "bbb", "c": "aaa"}) → {"a": "aaa", "b": "bbb", "c": "aaa"}