How to cleanup agent from GC 11g

Recently, I had have to clean up agents from  Em 11g, after deinstall procedure where agents resides,
 toy have to cleanup agents from Agents page in EM. How you can do this? It’s simple:

Step 1. Connect to OMS database at SYSMAN user.

Step 2. Locate name for agent and port:

select target_name from mgmt_targets where target_type='oracle_emd';

Step 3. Simple remove agent:

exec mgmt_admin.cleanup_agent('servername:3872');

In some cases I saw this:

*
ERROR at line 1:
ORA-20206: Target does not exist:
ORA-06512: at "SYSMAN.MGMT_ADMIN", line 630
ORA-06512: at "SYSMAN.MGMT_ADMIN", line 417
ORA-06512: at "SYSMAN.MGMT_ADMIN", line 1188
ORA-06512: at line 1

But re-run mgmt_admin.cleanup_agent help me, miracle :)