Weirdness in CHECK?

I just ran across this recent example:

If you perform the following, you get a truncated input:

test=# create table example (type char(5) NOT NULL);
CREATE
test=# insert into example VALUES ('VOLUME');
INSERT 156884 1
test=# select * from example;
type
-------
VOLUM
(1 row)

However, if you add CHECK in that checks for a string that is LONGER than
the CHAR(5), you get this:

test=# create table example(type char(5) NOT NULL CHECK (type IN
('MASS','VOLUME')));
CREATE
test=# insert into example VALUES ('VOLUME');
ERROR: ExecAppend: rejected due to CHECK constraint example_type

Is this correct behaviour? Perhaps it is, as CHECK is checking the
truncated value - but I just want to make sure it's not a bug!

Chris