ORA-01109: database not open

SQL> conn sys/oracle@pdb1 as sysdba
Connected.
SQL> SHOW CON_NAME

CON_NAME
------------------------------
PDB1

SQL> alter user system identified by oracle;
alter user system identified by oracle
                                *
ERROR at line 1:
ORA-01109: database not open


SQL> startup;
Pluggable Database opened.