ora 01531 after cold backup

I have a database that I do not have in archive log mode.   I schedule a cold backup via OEM and the backup seems to run fine but the following error shows up at the tail end of the backup log.  Any idea how I can correct this?

released channel: oem_backup_disk1

released channel: oem_backup_disk2

exit;

Recovery Manager complete.

startup open

SQL> SQL> Connected.

SQL> SQL> alter database open

*

ERROR at line 1:

ORA-01531: a database already open by the instance

My backup script is as follows.

SET COMPRESSION ALGORITHM 'BASIC' AS OF RELEASE 'DEFAULT' OPTIMIZE FOR LOAD TRUE;