SQL Server 2005 ENCRYPTBYKEY返回null

DECLARE @EmailEnc varbinary(maX)
declare @keyid uniqueidentifier
DECLARE @Email_test nvarchar(max)
set @Email_test = N'Sg@hotmail'
SELECT top 1 @keyid=[key_guid] FROM sys.symmetric_keys order by newid()
SET @EmailEnc=CAST(ENCRYPTBYKEY(@keyid,@Email_test) AS varbinary(max))
select @Email_test,@keyid,@EmailEnc
DECLARE @EmailEnc varbinary(maX)
declare @keyid