What are the limitations of Postgres master password in RDS?

According to RDS documentation (as of 2015.09.24), the limitation for all engines is

The password for the master database user can be any printable ASCII character except "/", """, or "@". Master password constraints differ for each database engine.

And, additionally, for PostgreSQL

Must contain 8 to 128 characters.

However, that's not correct. A password such as chZEYHssrb77AO9qHSXBU8rBIBuRJTTh does not work i.e. you may set the master password to it, but when trying to authenticate it fails. But chzeyhssrb77ao9qhsxbu8rbiburjtth does work.

Is it "lowercase characters" the only additional limitation or there are more?