BIBTEX - Help needed for adding ISBN in agsm bibliography style

Question

I am writing a report in a modified report class and am using agsm as a bibliography style. Using the help of:

hack for including doi in agsm bibliography style

I managed to add a DOI to my modified bibliography style. Now I wanted to do the same trick to add a ISBN to the book entry type.

In my modified .bst file I added in the ENTRY function isbn in between institution and journal. Then I added a function according the instructions of the link above.

FUNCTION {write.isbn}
    { isbn empty$
    { "" }
    { new.block "ISBN " isbn * }
    if$
}

then finally I modified the book entry to the following:

    FUNCTION {book}
    { output.bibitem
      list.label.output
      " \harvardyearleft " list.year * "\harvardyearright " * output.nonnull
      author empty$
      { editor "author and editor" item.check }
    { crossref missing$
    { "author and editor" editor either.or.check }
    'skip$
      if$
    }
  if$
  title.field field.used =
    { skip$ }
    { format.btitle "title" output.check }
  if$
  crossref missing$
    { format.bvolume output
      format.number.series output
      format.edition output
      publisher "publisher" output.check
      address output
    }
    { format.book.crossref output.nonnull
      format.edition output
    }
  if$
  new.block
  note output
  write.isbn output
  write.doi output
  fin.entry
  write.url
}

Again following the instructions of the link above. However, if I reran LaTeX and BibTeX (even multiple times), no isbn shows up. Can somebody help? Is there another way to make agsm show the ISBN or another 'hack' that may help?