\bbl@main@language error in biblatex

Question

Here is a simple document:

\documentclass{article}
\usepackage[%
  backend      = biber,
  bibstyle     = biblatex-sp-unified,
]{biblatex}
\addbibresource{test.bib}
\begin{document}

\cite{test}

\printbibliography{}
\end{document}

Which refers to this bbx and this simple bib file:

@article{test,
  Author = {test},
  Title = {test}
}

This used to work. After updating my MacTeX distribution this Sunday morning and (full disclosure) doing a sudo tlmgr update --all, I now receive errors like the following:

! Undefined control sequence.
<argument> \bbl@main@language

l.12 \printbibliography{
                        }
?

Curiously, if either the Author or Title field is missing, I don't get an error. Moreover, if I remove the bibstyle option, the error goes away. This leads me to suspect that the problem is somewhere in the bbx file linked above, but that's as far as my limited knowledge and expertise gets me.

I realize this might be a rather specific question (and I have opened an issue on the github I linked to), but perhaps there is a general issue causing the problem here, in which case others might benefit from a general solution.

Answer 1

This problem was fixed in version 3.5 of biblatex. An update to the current version of biblatex should solve the issue.

Note that a similar problem could occur with ancient versions of babel or polyglossia, in which case the solution is again to update these packages.

The culprit is the line

\DeclareFieldFormat[article,book,thesis,incollection,unpublished,inproceedings]{titlecase}{\MakeSentenceCase*{#1}}%

\MakeSentenceCase* checks for the document language as saved in \bbl@main@language, since you load neither babel nor polyglossia, no such command is available and biblatex complains.

The problem has been addressed in the dev version and a fix will be included in the upcoming version 3.5 of biblatex (see the discussion from 2016-06-19 on issue #393).

Of course the issue is not restricted to your special .bbx file - all code that uses \MakeSentenceCase* is affected. biblatex-ieee uses a special format for titles which calls that macro and so is also affected, even though you won't find a direct call for \MakeSentenceCase* in its \DeclareFieldFormat commands.

---

Solution one is to load babel or polyglossia with the correct language option (american or british for babel), this is especially recommended if you don't write in English.

If you don't want to load either of the packages, you can use the language-unaware unstarred version note that the starred version of \MakeSentenceCase* is normally strongly preferred because it obeys the language settings, so using the unstarred version can only be a temporary work-around

\DeclareFieldFormat[article,book,thesis,incollection,unpublished,inproceedings]{titlecase}{\MakeSentenceCase{#1}}%

or get rid of sentence case for good with

\DeclareFieldFormat[article,book,thesis,incollection,unpublished,inproceedings]{titlecase}{#1}%