ORA-44416 Unresolved Principal

Having an issue creating an ACL (Access Control List) in Oracle 11g database.

 BEGIN  
  DBMS_NETWORK_ACL_ADMIN.CREATE_ACL (  
   acl => 'resolve.xml',  
   description => 'Access Control',  
   principal => 'usera',  
   is_grant => TRUE,  
   privilege => 'connect');  
 END;  

I was receiving errors like this:

ERROR at line 1:
ORA-44416: Invalid ACL: Unresolved principal 'usera'
ORA-06512: at "SYS.DBMS_NETWORK_ACL_ADMIN", line 226
ORA-06512: at line 2

The solution is as simple as having the principal in uppercase!

   principal => 'USERA',