EncryptByKey Function Always Returning Null

When I use EncryptByKey function to encrypt text using AES 128 bit key, I get always null result. this is how I do the encryption:

ALTER PROCEDURE [dbo].[ProcMyProc](@ClearText nvarchar(50))
AS
BEGIN

OPEN SYMMETRIC KEY MyKey DECRYPTION BY PASSWORD= 'MY_Password_128';

Declare @Temp varbinary(8000);

Set @Temp =EncryptByKey(Key_GUID('MyKey'),@ClearText);

close symmetric key MyKey;

select @Temp as temp;
END


The result I get for this procedure is null. Is there something wrong with this code?