Linked Server And EXECUTE AS

I setup a linked server with the following server login mapping:

Local login: db1reader
Remote login: db2reader

db2reader is granted to have read permissions in db2.
I created a stored proc p1 using WITH EXECUTE AS 'db1reader'

Inside the stored proc, I access the link server db2 by "select * from db2server.db2.dbo.table"
db1 reader is granted to have EXECUTE permission on stored proc p1. Trustworthy is on in db1.

Then, I use SQL server 2005 management studio to connect to db1 using db1reader account. Try to execute the stored proc p1. It tells me "Access to the remote server is denied because no login-mapping exists."

Then, I launched a new query in management studio, type in the following TSQL


execute as login = 'db1reader'
go
select * from db2server.db2.dbo.table
revert
go

This time, it works without any error.

Then, in the same query window. I delete all my old TSQL statements and execute my p1 stored proc again.


USE [db1]
GO
DECLARE @return_value int
EXEC @return_value = [dbo].[p1]
SELECT 'Return Value' = @return_value
GO

This time, it works.

What did I do wrong?