diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index 407d87a34f..eb65f46986 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -797,7 +797,7 @@ function simpletest_script_run_one_test($test_id, $test_class) {
       // This also prevents mock objects in PHPUnit 6 triggering silenced
       // deprecations from breaking the test suite. We should consider changing
       // this to 'strict' once PHPUnit 4 is no longer used.
-      putenv('SYMFONY_DEPRECATIONS_HELPER=weak_vendors');
+      putenv('SYMFONY_DEPRECATIONS_HELPER=strict');
     }
     if (is_subclass_of($test_class, TestCase::class)) {
       $status = simpletest_script_run_phpunit($test_id, $test_class);