how to add signature to a DDL trigger?

I have a DDL trigger on a database. I want to add a signature to it, by certificate. I have full sa rights, but when I try to run the statement

ADD SIGNATURE TO OBJECT::[objects_monitoring]
BY CERTIFICATE [mail_certificate] 
WITH PASSWORD =开发者_运维百科 'password1';
GO

I get the following error

Msg 15151, Level 16, State 1, Line 2
Cannot alter the object 'objects_monitoring', because it does not exist or you do not have permission.

How do I go about adding the signature to this DDL trigger?

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According to this link, DDL triggers cannot be signed.