BibTeX: I didn't find a database entry for "" -- empty database entry?

Question

I am working on a Latex template and I keep getting the following error:

BibTeX: I didn't find a database entry for ""

At first I though something in my bib had an unexpected empty citation, but there is none. I've also tried to rename the bib file, no luck... any ideas? I've look everywhere and I can't find a solution yet. Any help is appreciated.

Answer 1

This may be caused by duplication of database files in the command \bibliography{}.

The bibtex command will produce the following output

This database file appears more than once: foo.bib
---line 4 of file Untitled2.aux
 : \bibdata{foo,foo
 :                 }
I'm skipping whatever remains of this command
Database file #1: foo.bib
(There was 1 error message)

if you write in your tex file as

\bibliography{foo2022, foo2022}

Such an error won't cause any trouble if the duplication appears as the last database file. Unfortunately, if you append another database file after that duplicated one, as suggested by the error message, the new file is ignored so bibtex can't find that database entry.

So review the error message from bibtex and exclude the duplicated database files.

I have prepared some tex and bib files for you to reproduce such an error.

tex file

\documentclass[a4paper,12pt,oneside]{report} 
\usepackage[round]{natbib}
\bibliographystyle{plainnat}

\usepackage[textheight=23.7cm,
            hmargin=26mm,
            top=26mm, 
            headheight=0.5cm,
            headsep=0.5cm,
           ]{geometry}

\renewcommand{\bibname}{References}

\begin{document}
\citep{foo2022}, \citep{foo2:foo2022}

\bibliography{foo,foo,foo2}
\end{document}

foo.bib

@article{foo2022,
        author = {Hello, World},
        journal = {Journal},
        month = {01},
        number = {1},
        pages = {1-10},
        title = {Title},
        volume = {100},
        year = {2022}}

foo2.bib

@article{foo2:foo2022,
        author = {Hello, World},
        journal = {Journal},
        month = {01},
        number = {1},
        pages = {1-10},
        title = {Title},
        volume = {100},
        year = {2022}}

To compile, use the following commands

pdflatex <your file>
bibtex <your file>
pdflatex <your file>
pdflatex <your file>

To correct the above-mentioned error, change

\bibliography{foo,foo,foo2}

to

\bibliography{foo,foo2}

Answer 2

It could be possible there's a non-display character (like a zero width space, for example) that got into one of your citations.

Try doing a find command for "cite{" and compare it with the number of results for "cite". If there's a difference then you've found the citation with a stray character in it, and you can just copy a properly formatted string and paste in place.

There could also be such a character within after the bracket which will cause the same problem, but that will be a bit harder to find.

I thought of this thanks to this question where the author also had a problem with a non-displaying character getting into his citation. https://tex.stackexchange.com/questions/33416/bibtex-i-didnt-find-a-database-entry-for