Service Broker and procedure which contains Execute as

We have queue on Service Broker:

ALTER QUEUE [dbo].[my_queue]WITH
        STATUS =ON,
        RETENTION =OFF,
        ACTIVATION (
                      STATUS =ON,
                      PROCEDURE_NAME =[dbo].[my_detect_event],
                      MAX_QUEUE_READERS =1,EXECUTEAS N'dbo'),
        POISON_MESSAGE_HANDLING (STATUS =OFF)

Procedure has code (i cut some text with RECEIVE, because it's not linked to the issue):

ALTERPROCEDURE dbo.my_detect_eventASBEGIN......cut text with RECEIVE TOP(1) message_body FROM my_queue INTO@NotificationStore--EXECUTE AS LOGIN = 'user_sa'INSERTINTO dbo.log( time, text)VALUES( GETDATE(),'test')......END

user_sa - has SA rights.

If the service fires an event, then the table data is inserted successfully.

But if you uncomment the line EXECUTE AS LOGIN = 'user_sa', then appears the error:

Cannot execute as the server principal because the principal "user_sa" does not exist, this type of principal cannot be impersonated, or you do not have permission.

Pls help to find solutions. Thanks in advance!

Technical info: Microsoft SQL Server 2012 - 11.0.5582.0 (X64) Enterprise Edition