How should the DOCTYPE statement appear when saving as XML?

This seems so trivial, but I cannot figure it out.

When I save a Frame file as XML, the DOCTYPE statement is written as

<?DOCTYPE ..... ?>

instead of

<!DOCTYPE ..... >

I noticed this because Frame exports the content of empty table cells as the nobreaking space entity. When I try to open the XML file in a browser, the browser cannot parse the nobreaking space entity, even if I include its definition in the ?DOCTYPE statement. Changing the DOCTYPE statement to <!DOCTYPE ... > makes the ENTITY definition visible to the browser.

What am I missing?

Thanks,

Van