Hash index on partial char field not working

eval create table t1 (

c char(10),

unique key a using hash (c(1))

) engine=myisam;

show create table t1;

insert into t1 values ('б');

--error ER_DUP_ENTRY

insert into t1 values ('бб');

--error ER_DUP_ENTRY

insert into t1 values ('ббб');

drop table t1;

mysqltest: At line 9: query 'insert into t1 values ('бб')' succeeded - should have failed with errno 1062...