Open Encryption Key In Stored Procedure

Hi,
I would like to create a stored procedure, which accept RunAsUser, MasterKeyPassword and also ASymmetricKeyPassword. In this stored procedure, it call OPEN MASTER KEY and etc. I put these code in stored procedure so that my support staff can call it when doing maintenance by passing the parameter. I don't expect them to remember OPEN MASTER KEY and etc syntax.

However, I hit error "Invalid Syntax" when I run my code as below. Any ideas?

Thank you

-- ================================================
-- Template generated from Template Explorer using:
-- Create Procedure (New Menu).SQL
--
-- Use the Specify Values for Template Parameters
-- command (Ctrl-Shift-M) to fill in the parameter
-- values below.
--
-- This block of comments will not be included in
-- the definition of the procedure.
-- ================================================
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author: <Author,,Name>
-- Create date: <Create Date,,>
-- Description: <Description,,>
-- =============================================
CREATE PROCEDURE GrantMe
-- Add the parameters for the stored procedure here
@RunAsUser varchar(20), @MasterKey varchar(30), @ASKey varchar(30)
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;

EXEC AS USER=@RunAsUser;

OPEN MASTER KEY DECRYPTION BY PASSWORD=@MasterKey;

OPEN SYMMETRIC KEY HRMS
DECRYPTION BY ASYMMETRIC KEY FlexHRMS WITH PASSWORD=@AsKey ;
END
GO