64-bit ODAC 11.2 Release 5 (11.2.0.3.20) NullReferenceException while Close()

Hi All,
Our application uses 64-bit ODAC 11.2 Release 5 (11.2.0.3.20) on Windows Server 2008 R2.
We experience intermittent NullReferenceException while closing the connection.

Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.NullReferenceException
Stack:

   at Oracle.DataAccess.Client.ConnectionDispenser.CopyPooledConCtx(Oracle.DataAccess.Client.OpoConValCtx* ByRef, Oracle.DataAccess.Client.OpoConValCtx*)
   at Oracle.DataAccess.Client.ConnectionPool.PutConnection(Oracle.DataAccess.Client.OpoConCtx ByRef, Boolean, Boolean, Boolean, Int32)
   at Oracle.DataAccess.Client.ConnectionDispenser.Close(Oracle.DataAccess.Client.OpoConCtx ByRef, Boolean)
   at Oracle.DataAccess.Client.OracleConnection.Close()
   at <application modules are coming>




Thanks in advance for any help.