Could Not Update Object When One Unique Key is NULL.

When i am integrating MongoDB with my java Aplication and MAP of java support NULL as unique key then why i am unable to save my Object.

NOTE: Null key can be a business senario for some organisation

To collect leftover data.

Try to run below main program in java

public static void main(String[] args)
{
       Map<String, String> strMap= new HashMap<>();
        strMap.put(null,"Hello World");
        System.out.print(strMap);
}