Use of pdflatex -fmt option

Transitioning from manual compilation with pdflatex to rubber, I believe I fail to understand how to properly pass the -fmt option to pdflatex.

pdflatex workflow

My goal is to load a "precompiled" version of the document preamble, which I normally do with -fmt.

  1. write preamble (everything prior to \begin{document}) to preamble.tex
  2. write main document (everything else) to main.tex
  3. "precompile" the preamble with the command: pdflatex -ini -jobname="main" "&pdflatex preamble.tex\dump"
  4. generate the pdf document with pdflatex: pdflatex -fmt="main" "main"

See attached sources for a minimal working example: preamble.tex, main.tex, compilation.sh

Attempt with rubber

Steps 1-3 are unchanged.

For pdf production I tried rubber -m pdftex:"-fmt=main" main but receive the following error message, indicating that the preamble has not been loaded properly:

main.tex:2: The font size command \normalsize is not defined:
main.tex:2: leading text: \begin{document}
main.tex:10: Undefined control sequence \eminem.
main.tex:10: leading text: \eminem
main.tex:11: The font size command \normalsize is not defined:
main.tex:11: leading text: \end{document}

Am I missing something here?