How to export a single subtree but include document preamble?

["Richard H. Stanton" <rhstanton@berkeley.edu>]

I just tried to export a single subtree from an org document to a PDF file:

1) Type  C-c C-e [org-export-dispatch]
2) C-s to narrow to subtree
3) l -> o to export to PDF via LaTeX

However, instead of exporting the portion of the file I want, I get the error message

Undefined Org macro: newline; aborting

That macro is defined in a setup file that is #+included at the top of the file, so exporting the entire file works fine.

I assume the problem is that when I export only one subtree, the setup file is not #+included, and I can see it sort of makes sense, but it would be nice if there were an option to include one or more lines from the preamble when exporting just a subtree, since otherwise exporting a subtree doesn’t actually work! Is there a simple way to do this?

Thanks.