Simple MySQL Index

I have a single table which I would like to search on a unique column
varchar(15) that may have some nulls. Employee social club member no,
some employees have a number and others don't. Those that do have a number
will all have a unique number.
>
I had added an index using:
ALTER TABLE employees ADD INDEX(emp_socialclubno);
>
However when I run:
mysqlexplain select employeeid from employees where
emp_socialclubno=103833988;
+----+-------------+------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len |
ref | rows | Extra |
+----+-------------+------------+------+---------------+------+---------+------+--------+-------------+
| 1 | SIMPLE | properties | ALL | emp_socialclubno | NULL | NULL |
NULL | 170361 | Using where |
+----+-------------+------------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)
>
So this indicates that even though the emp_socialclubno column has an index
it is not being used when this column is searched ?
>
Is this right or am I missing something ?
>
Thanks in advance...