Try following test:
create user test_expire identified by abcdefgh;
grant create session to test_expire;
alter user test_expire password expire;
Then, login this user, you will see change pass dialog. Try to set one (1) character log password. You can’t!.
When your pass was one character long before expiration, everything works fine, you can change on one character pass too. When this pass was longer than 1 char, Your new pass must have at least 2 chars long.