bjnano.bst causes "You can't pop an empty literal stack for entry XXX" error for @Inproceedings

Question

I'm using the beilstein.cls class from the beilstein package to format a document. The references in these documents are formatted using the bjnano.bst style file that comes with the beilstein package from CTAN (https://ctan.org/pkg/beilstein). In my document I'm referencing an InProceedings-type reference, which causes the following error with bibtext:

This is BibTeX, Version 0.99d (MiKTeX 20.7)
The top-level auxiliary file: co-paper-MWE.aux
The style file: bjnano.bst
Database file #1: MWE.bib
You can't pop an empty literal stack for entry 5420955
while executing---line 1463 of file bjnano.bst
(There was 1 error message)

In spite of the error, a proper bibliography is created and the entry seems to be completely there. I had a look at line 1463 of the style file, however I do not understand the way the formatting works.

Here is the corresponding MWE:

\documentclass{beilstein}
\usepackage{filecontents}
\begin{filecontents*}{MWE.bib}
@INPROCEEDINGS{5420955,
    author={J. J. {Martin} and V. {Neburchilov} and H. {Wang} and W. {Qu}},
    booktitle={2009 IEEE Electrical Power Energy Conference (EPEC)}, 
    title={Air cathodes for metal-air batteries and fuel cells}, 
    year={2009},
    volume={},
    number={},
    pages={1-6},
    publisher={{IEEE}},
    doi = {10.1109/epec.2009.5420955},}
\end{filecontents*}

\begin{document}
    Text\cite{5420955} 
    \bibliography{MWE}
\end{document}

Does the error exist within the style file or does my .bib file contain an error?

EDIT: As pointed out by the package maintainer in an answer below, this bug exists in version 2.0 (dated 2020-02-26) and will be fixed with a new version update.

Answer 1

There is a small bug in the inproceedings function in bjnano.bst.

After printing the booktitle the function calls add.semicolon write$ (line 1121). At this point apparently the stack is empty, therefore the error occurs. Replacing the line with new.block (which also prints a semicolon) the error disappears, while the output remains the same.

Full function without the error:

FUNCTION {inproceedings}
{ output.bibitem
  format.authors "author" output.check
  after.item 'output.state :=
  title empty$
    'skip$
    { title format.title.noemph output
      after.sentence 'output.state :=
    }
  if$
  booktitle format.proc.title.in "booktitle" output.check
  new.block
  format.pub.address "publisher" output.check
  format.bdate "year" output.check
  pages empty$
    'skip$
    { new.block
      format.pages output }
  if$
  format.doi output
  fin.entry
}

Answer 2

Thanks for the report. An updated version 2.1 has just been sent to CTAN.