As we know the 1st argument of ELT() function must be an integer value but when we provide index number which is not an integer the MySQL ELT() function returns NULL with a warning.
Example
mysql> select ELT('one','Ram,is,good,boy')As Result;
+--------+
| Result |
+--------+
| NULL |
+--------+
1 row in set, 1 warning (0.00 sec)
mysql> Show Warnings;
+---------+------+------------------------------------------+
| Level | Code | Message |
+---------+------+------------------------------------------+
| Warning | 1292 | Truncated incorrect INTEGER value: 'one' |
+---------+------+------------------------------------------+
1 row in set (0.00 sec)