Vignette failing to build with no code changes

I’m getting the error:

Running ‘texi2dvi’ on ‘dirichletprocess.tex’ failed.
LaTeX errors:
! Undefined control sequence.
l.1168 \ExplSyntaxOn

After a monthly scheduled build, having not pushed any changes to my package or .travis.yml. So I think something has changed in the TeX installation that is used.

Failing build: https://travis-ci.org/github/dm13450/dirichletprocess/jobs/700188538
Working build: https://travis-ci.org/github/dm13450/dirichletprocess/builds/700177807

Any help or pointers is greatly appreciated!

Managed to fix it:

\usepackage{expl3}

was needed in my vignette.

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