help with index on table

[Eitan Gur]

Hi

I tried to create an index on one of my tables.
After creating this index, the queries return an empty set, instead of the 
expected result.

I have a table user looks like this:
id - int(11)
username - varchar(250)
address - varchar(250)
more columns...

After creating an index:
CREATE INDEX my_index on user (username(10));

I tried to run:
SELECT * FROM user WHERE username like 'homer';

I get an empty set, however when I run the same query without the index - I get 
results.

What am I doing wrong?

Thanks.