Can't use multi-line runners.pre_build_script

Summary

I'm trying to use a multi-line YAML script for the pre_build_script configuration item in /etc/gitlab-runner/config.toml. According to the documentation, this is legal.

If I use the triple-quoted string with line-breaks, or a single-quoted string with line continuations, I get the error on startup:

FATAL: Near line 13 (last key parsed 'runners.pre_build_script'): Invalid escape character '\n'. Only the following escape characters are allowed: \b, \t, \n, \f, \r, \", \/, \\, \uXXXX and \UXXXXXXXX.

If I put everything on one line, it works, but it's very difficult to read. ;-)

Environment description

Private instance, using the Kubernetes executor (though NA here).

Used GitLab Runner version

Using Docker image gitlab/gitlab-runner:v9.1.1.