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:
Working build:

Any help or pointers is greatly appreciated!

Managed to fix it:


was needed in my vignette.