How do I use AUCTeX to perform forward search/synchronization in an included document?

I’m trying to sync my .tex file with the .pdf output with a forward search on OS X using Emacs + AUCTeX and Skim.app. I have added the following code to my Emacs configuration:

(setq TeX-auto-save t)
(setq TeX-parse-self t)
(setq-default TeX-master nil)
(add-hook 'LaTeX-mode-hook 'visual-line-mode)
(add-hook 'LaTeX-mode-hook 'flyspell-mode)
(add-hook 'LaTeX-mode-hook 'LaTeX-math-mode)
(add-hook 'LaTeX-mode-hook 'turn-on-reftex)
(setq reftex-plug-into-AUCTeX t)
(setq TeX-PDF-mode t)

;; Use Skim as viewer, enable source <-> PDF sync
;; make latexmk available via C-c C-c
;; Note: SyncTeX is setup via ~/.latexmkrc (see below)
(add-hook 'LaTeX-mode-hook (lambda ()
                             (push
                              '("latexmk" "latexmk -pdf %s" TeX-run-TeX nil t
                                :help "Run latexmk on file")
                              TeX-command-list)))
(add-hook 'LaTeX-mode-hook (lambda ()
                             (push
                              '("pdflatex" "pdflatex -synctex=1 %s" TeX-run-TeX nil t
                                :help "Run pdflatex with SyncTex on file")
                              TeX-command-list)))
(add-hook 'TeX-mode-hook '(lambda () (setq TeX-command-default "pdflatex")))

;; use Skim as default pdf viewer
;; Skim's displayline is used for forward search (from .tex to .pdf)
;; option -b highlights the current line; option -g opens Skim in the background
(setq TeX-view-program-selection '((output-pdf "PDF Viewer")))
(setq TeX-view-program-list
      '(("PDF Viewer" "/Applications/http://ift.tt/1s8Sazk -b -g %n %o %b")))

(add-hook 'LaTeX-mode-hook
          (lambda () (local-set-key (kbd "<M-S-mouse-1>") #'TeX-view)))

(setenv "PATH" (concat "/usr/texbin" ":" "/Applications/http://ift.tt/1FEtibk" ":" "/usr/local/bin" ":" (getenv "PATH")))
(setq exec-path (append
                 '("/usr/texbin"
                   "/Applications/http://ift.tt/1vv4eQZ"
                   "/Applications/http://ift.tt/1FEtkjq"
                   "/usr/local/bin") exec-path))

Everything works fine in the main document, however, when I try to use forward search in an included file, say chapter1.tex, I get an error:

Output file "chapter1.pdf" does not exist.