CREATE PROCEDURE [dbo].[test]
WITH EXECUTE AS '_test'
AS 
BEGIN
	print SUSER_NAME()
	execute ('select 1') as login='_test'
END

go

exec test
go

_test
Msg 15406, Level 16, State 1, Procedure test, Line 6
Cannot execute as the server principal because the principal "_test" does not exist, this type of principal cannot be impersonated, or you do not have permission.

execute ('select 1') as login='_test'