Session with event enq: tm contention with no blocking session in v$session

Hi.
I´m having a problem in my production environment. There is some sessions with event "enq: tm contention" but there is no blocking sessions in v$session as you can see below:

1 - session with enq: tm contention event

Connected to Oracle Database 11g Enterprise Edition Release 11.2.0.2.0
Connected as asavioli
 
SQL> select SID, USERNAME, STATUS, EVENT  from v$session where SID=485;
 
       SID USERNAME                       STATUS   EVENT
---------- ------------------------------ -------- ----------------------------------------------------------------
       485 NPTCADM                        ACTIVE   enq: TM - contention

2 - This session has no other session blocking it. How can happen a enq: tm contention this way??
 
SQL> select SID, USERNAME, BLOCKING_SESSION from v$session where SID=485;