Rules / Output

LaTeX Warning: Citation '…' undefined — Undefined citation

A cited key has no bibliography entry, so the PDF shows the key or [?].

Fix

Fix the key or add the entry to the .bib file, then rebuild; or remove the \cite if the source is not known.

Example

Reproduced on TeX Live 2026 by CI · lualatex · corpus/biblatex-undefined-citation

Citation key missing from the .bib file (biblatex)

Fails: main.tex
\documentclass{article}
\usepackage{biblatex}
\addbibresource{references.bib}
\begin{document}
Typesetting is a solved problem \autocite{knuth1984}, as is
literate programming \autocite{knuth92}.
\printbibliography
\end{document}
Fixed
\documentclass{article}
\usepackage{biblatex}
\addbibresource{references.bib}
\begin{document}
Typesetting is a solved problem \autocite{knuth1984}.
\printbibliography
\end{document}
What TeX prints in main.log
LaTeX Warning: Citation 'knuth92' on page 1 undefined on input line 6.