I am working fine on my Overleaf document until I inserted citation when the following error appears after using \printbibliography to generate reference list.:

Undefined control sequence.                      main.tex line 60
The compiler is having trouble understanding a command you have used. Check that the command is spelled correctly. If the command is part of a package, make sure you have included the package in your preamble using \usepackage{...}.
namepartfamily ->Zim\x 
                        {fffd}\x {fffd}nyi
l.60 \end
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.

This error disappears when \printbibliography is commented out, but then no reference list generated. I can confirm I added all required packages. See the document below:

\usepackage[top=25mm, bottom=25mm, left=25mm, right=25mm]{geometry}


    %Table of content

%Abasract page
    \textbf{\textit Abstract} 

    This is a simple one-page abstract template. Please keep your abstract length at one page. The abstract should be in English.

%begin main document

    \section{Statement of Problem}
    This is the beginning of the introduction section.
    %\subsection{Research Questions}

    \section{Research Objectives}

    \section{Pre-Thesis Structure}

\chapter{Fundamental Concepts}

\chapter{State of the Art}

\chapter{Material and Methods}

\chapter{Pilot Study}

%=== Reference Lists ====

  • 1
    Sounds like some character problem. Check in your .bib file and search for the text Zim, and check the character after it: you might need to re-type it carefully e.g. Zimányi or Zim\'{a}nyi (or similar—Zimányi was the top search result I got when googling Zim.?nyi on Google Scholar) – LianTze Lim Aug 19 at 1:28
  • 1
    Yes, seems as though Zim?ny is malformed. If LianTzeLim's guess that it should be Zimányi is correct, the issue is probably that the á is not encoded as Biber/biblatex expects. The easy solution would be that your .bib file is UTF-8, but LaTeX expects US-ASCII. Did you try adding \usepackage[utf8]{inputenc} to your document? But maybe the á is malformed. Did you try deleting and re-writing the name? In any case you should clear the cache after you made the changes to avoid errors hanging about in temporary files: overleaf.com/learn/how-to/Clearing_the_cache – moewe Aug 19 at 5:01
  • Ah, you're right. adding the package \usepackage[utf8]{inputenc} fix the problem. – arilwan Aug 19 at 10:22

Your .bib file probably contains a line like

author  = {József Zimányi},

(all credits for the detective work of figuring out the name go to LianTze Lim in the comments). Quite likely your .bib file is encoded in UTF-8 or another non-ASCII encoding.

But with your preamble (and the outdated TeX system on Overleaf) LaTeX expects US-ASCII input only (a more current LaTeX will use UTF-8 as default encoding). The easiest way to get things to work again is to tell LaTeX you want to use UTF-8 with


You may have to clear the cache (see https://www.overleaf.com/learn/how-to/Clearing_the_cache) before you can recompile cleanly after that, but then you should get the desired output.

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