How can you add an Attribute with NULL-Value

Hello,

i want to add an Attribute which has a NULL-Value. How can you do that with the library?

SmlDocument doc = new SmlDocument("Message");  
doc.Root.AddAttribute("From", "-");

This code will add the hyphen to the document but of course with quotationmarks included. I get this:

MESSAGE
      FROM "-"
END

But i want this:

MESSAGE
      FROM -
END

Thank You!

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