Bibliography: You can't pop an empty literal stack for entry

Question

I am using a custom .bst file for thesis write up and getting the following error: You can't pop an empty literal stack for entry.

\documentclass[a4paper,12pt]{book}
\usepackage[sort,round]{natbib}
\usepackage{bibunits}
\begin{document}

This is a citation \cite{Chen1976}.

\bibliographyunit
\bibliographystyle{bibstyle/noon}
\bibliography{bib/mybib}

\end{document}

These is the entry:

@Article{Chen1976,
  author    = {Peter P. Chen},
  title     = {The Entity-Relationship Model - Toward a Unified View of Data},
  journal   = {{ACM} Trans. Database Syst.},
  year      = {1976},
  volume    = {1},
  number    = {1},
  pages     = {9--36},
  bibsource = {dblp computer science bibliography, https://dblp.org},
  biburl    = {https://dblp.org/rec/journals/tods/Chen76.bib},
  doi       = {10.1145/320434.320440}
}

The problem is that each entry is rendered in the output PDF like this:

[CHEN (1976), ]

As you can see, after the date there is always a comma followed by a white space.

I believe this is related to the following warning I receive when compiling:

Process started: bibtex.exe "thesis"

This is BibTeX, Version 0.99d (MiKTeX 2.9.7250 64-bit)
The top-level auxiliary file: thesis.aux
The style file: bibstyle/noon.bst
A level-1 auxiliary file: bu.aux
Database file #1: bib/mybib.bib          
You can't pop an empty literal stack for entry Chen1976
while executing---line 1558 of file bibstyle/noon.bst
You can't pop an empty literal stack for entry everyentry
while executing---line 1558 of file bibstyle/noon.bst

The .bst file is this one.

Am I missing something?

EDIT: I noticed that only a subset of entries receive the warning. Here a couple:

@inproceedings{TomanW09,
    Author = {David Toman and Grant E. Weddell},
    Booktitle = {{IJCAI} 2009, Proceedings of the 21st International Joint Conference on Artificial Intelligence},
    Date-Added = {2017-02-05 16:40:40 +0000},
    Date-Modified = {2017-02-05 16:41:30 +0000},
    Optbdsk-Url-1 = {http://ijcai.org/Proceedings/09/Papers/161.pdf},
    Optbibsource = {dblp computer science bibliography, http://dblp.org},
    Optbiburl = {http://dblp.uni-trier.de/rec/bib/conf/ijcai/TomanW09},
    Opturl = {http://ijcai.org/Proceedings/09/Papers/161.pdf},
    Pages = {948--954},
    Timestamp = {Wed, 20 Jul 2016 14:20:40 +0200},
    Title = {Applications and Extensions of {PTIME} Description Logics with Functional Constraints},
    Year = {2009}}

@InProceedings{racer-a,
  author    = {Volker Haarslev and Ralf M{\"{o}}ller},
  title     = {Description of the {RACER} System and its Applications},
  booktitle = {Working Notes of the 2001 International Description Logics Workshop (DL-2001), Stanford, CA, USA, August 1-3, 2001},
  year      = {2001},
  editor    = {Carole A. Goble and Deborah L. McGuinness and Ralf M{\"{o}}ller and Peter F. Patel{-}Schneider},
  volume    = {49},
  series    = {{CEUR} Workshop Proceedings},
  publisher = {CEUR-WS.org},
  bibsource = {dblp computer science bibliography, https://dblp.org},
  biburl    = {https://dblp.org/rec/bib/conf/dlog/HaarslevM01},
  url       = {http://ceur-ws.org/Vol-49/HaarslevMoeller-132start.ps},
}

Answer 1

noon.bst uses lines like

"[" write$  short.list write$ " " write$ format.date "year" output.check write$ "] " write$

to print the citation label at the beginning of the bibliography again.

The problematic bit is

format.date "year" output.check write$

If there is something to print output.check will already perform a write$ itself, so we may end up with essentially

format.date write$ write$

in many situations. That causes the error you report.

Apparently, someone noticed this issue and removed the problematic bit from some of the FUNCTIONs already, so in some instances we find

"[" write$  short.list write$ " " year * write$ "] " write$

But they missed or ignored a few occurrences.

---

I suggest a slightly different approach where we externalise the label generation to a FUNCTION that mimics the way the actual citation label is produced.

You can get fixed version of noon.bst at https://gist.github.com/moewew/a4d48dd54655a610ae9b68522a5f48aa. The diff to the original noon.bst from https://textuploader.com/1qfzj is

--- orig-noon.bst   2020-04-29 08:44:11.162162900 +0200
+++ noon.bst    2020-04-28 22:18:25.871804100 +0200
@@ -982,9 +982,26 @@
 { organization "organization" bibinfo.check format.org.or.pub
 }

+FUNCTION {bib.label}
+{
+  "["
+  short.list
+  *
+  " "
+  *
+  year duplicate$ empty$
+  short.list key field.or.null = or
+     { pop$ "" }
+     'skip$
+  if$
+  *
+  "] \newblock"
+  *
+}
+
 FUNCTION {article}
 { output.bibitem
-  "[" write$  short.list write$ " " year * write$ "] " write$
+  bib.label write$ newline$
   format.authors "author" output.check
   author format.key output
   new.block
@@ -1008,7 +1025,7 @@
 }
 FUNCTION {book}
 { output.bibitem
-  "[" write$  short.list write$ " " year * write$ "] " write$
+  bib.label write$ newline$
   author empty$
     { format.editors "author and editor" output.check
       editor format.key output
@@ -1042,7 +1059,7 @@
 }
 FUNCTION {booklet}
 { output.bibitem
-  "[" write$  short.list write$ " " year * write$ "] " write$
+  bib.label write$ newline$
   format.authors output
   author format.key output
   new.block
@@ -1058,7 +1075,7 @@

 FUNCTION {inbook}
 { output.bibitem
-  "[" write$  short.list write$ " " write$ format.date "year" output.check write$ "] " write$
+  bib.label write$ newline$
   author empty$
     { format.editors "author and editor" output.check
       editor format.key output
@@ -1098,7 +1115,7 @@

 FUNCTION {incollection}
 { output.bibitem
-  "[" write$  short.list write$ " " write$ format.date "year" output.check write$ "] " write$
+  bib.label write$ newline$
   format.authors "author" output.check
   author format.key output
   new.block
@@ -1125,41 +1142,41 @@
 }
 FUNCTION {inproceedings}
 { output.bibitem
-  "[" write$  short.list write$ " " write$ format.date "year" output.check write$ "] " write$
-  format.authors "author" output.check
-  author format.key output
-  new.block
-  format.title "title" output.check
-  new.block
-  crossref missing$
-    { format.in.ed.booktitlenoit "booktitle" output.check
-      format.bvolumenoit output
-      format.number.series output
-      format.pages output
-      new.sentence
-      publisher empty$
-        { format.organization.address output }
-        { organization "organization" bibinfo.check output
-          format.publisher.address output
-        }
-      if$
-      %format.date "year" output.check
-     year write$ ", " write$ 
-    }
-    { format.incoll.inproc.crossref output.nonnull
-      format.pages output
-      %format.date "year" output.check
-     year write$ ", " write$ 
-    }
+  new.block
+  bib.label write$ newline$
+  format.authors "author" output.check
+  author format.key output
+  new.block
+  format.title "title" output.check
+  new.block
+  crossref missing$
+    { format.in.ed.booktitlenoit "booktitle" output.check
+      format.bvolumenoit output
+      format.number.series output
+      format.pages output
+      new.sentence
+      publisher empty$
+        { format.organization.address output }
+        { organization "organization" bibinfo.check output
+          format.publisher.address output
+        }
+      if$
+      %format.date "year" output.check
+  year write$ ", " write$ 
+    }
+    { format.incoll.inproc.crossref output.nonnull
+      format.pages output
+      %format.date "year" output.check
+  year write$ ", " write$ 
+    }
   if$
-  new.block
   format.note output
   fin.entry
 }
 FUNCTION {conference} { inproceedings }
 FUNCTION {manual}
 { output.bibitem
-  "[" write$  short.list write$ " " year * write$ "] " write$
+  bib.label write$ newline$
   format.authors output
   author format.key output
   new.block
@@ -1176,7 +1193,7 @@

 FUNCTION {mastersthesis}
 { output.bibitem
-  "[" write$  short.list write$ " " year * write$ "] " write$
+  bib.label write$ newline$
   format.authors "author" output.check
   author format.key output
   new.block
@@ -1194,8 +1211,7 @@

 FUNCTION {misc}
 { output.bibitem
-%  "[" write$  short.list write$ " " year * write$ "] " write$
-  "[" write$  short.list write$  "] " write$
+  bib.label write$ newline$
 %  format.authors output
 %  author format.key output
   new.block
@@ -1209,7 +1225,7 @@
 }
 FUNCTION {phdthesis}
 { output.bibitem
-  "[" write$  short.list write$ " " year * write$ "] " write$
+  bib.label write$ newline$
   format.authors "author" output.check
   author format.key output
   new.block
@@ -1227,7 +1243,7 @@

 FUNCTION {proceedings}
 { output.bibitem
-  "[" write$  short.list write$ " " year * write$ "] " write$
+  bib.label write$ newline$
   format.editors output
   editor format.key output
   new.block
@@ -1249,7 +1265,7 @@

 FUNCTION {techreport}
 { output.bibitem
-  "[" write$  short.list write$ " " year * write$ "] " write$
+  bib.label write$ newline$
   format.authors "author" output.check
   author format.key output
   new.block
@@ -1267,7 +1283,7 @@

 FUNCTION {unpublished}
 { output.bibitem
-  "[" write$  short.list write$ " " year * write$ "] " write$
+  bib.label write$ newline$
   format.authors "author" output.check
   author format.key output
   new.block
@@ -1384,6 +1400,7 @@
   'label :=
 }

+
 FUNCTION {sort.format.names}
 { 's :=
   #1 'nameptr :=