Social Networking Friends table - Explain Plan - Error Failed to enable constraints error

I have a below table structure for friends 开发者_高级运维table

TABLE STRUCTURE

CREATE TABLE `my_friends` (
  `first_user_id` int(10) unsigned NOT NULL,
  `second_user_id` int(10) unsigned NOT NULL,
  `approved` tinyint(1) NOT NULL DEFAULT '0',
  `request_sent` int(10) unsigned DEFAULT NULL,
  `request_approved` int(10) unsigned DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

and

DATA

insert into `my_friends` values (4,1,1,null,null);
insert into `my_friends` values (7,8,1,null,null);
insert into `my_friends` values (1,7,1,null,null);
insert into `my_friends` values (2,7,1,null,null);
insert into `my_friends` values (4,2,1,null,null);

To get all the friends for user id "1" , I am running the below query

QUERY TO GET FRIENDS

SELECT  first_user_id user_id, second_user_id friend_id  from my_friends where first_user_id=1 and approved=1
union
SELECT  second_user_id user_id ,first_user_id friend_id from my_friends where second_user_id=1 and approved=1

and when I check explain plan for this , this throws

EXPLAIN PLAN ERROR

Error
Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.

I am using this query in most of the queries as a subquery and this one causing the issue to check explain plan for all other queries

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This is an issue with Mysql and raised with mysql