diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 21d9f57..2e3ab1f 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -34,4 +34,11 @@ variables:
   OPT_IN_TEST_NEXT_MINOR: '1'
   OPT_IN_TEST_NEXT_MAJOR: "0"
   OPT_IN_TEST_MAX_PHP: "1"
+  # The phpunit binary (_PHPUNIT_CONCURRENT: 0) loads phpunit.xml from the
+  # project root. That file is the scaffold config for local and GitHub runs.
+  # This flag points the binary at the DrupalCI config instead.
   _PHPUNIT_EXTRA: '--no-coverage --configuration=phpunit.gitlab-ci.xml'
+  # run-tests.sh (_PHPUNIT_CONCURRENT: 1, the default) uses core's phpunit
+  # config and needs no extra options. The '-' stops the template copying
+  # _PHPUNIT_EXTRA into this variable.
+  _RUNTESTS_EXTRA: '-'