diff --git a/lib/simpletest/testfilterconfig.php b/lib/simpletest/testfilterconfig.php
index 28ad2df56f9..379fc1197e5 100644
--- a/lib/simpletest/testfilterconfig.php
+++ b/lib/simpletest/testfilterconfig.php
@@ -785,6 +785,7 @@ class filter_filter_set_applies_to_strings extends UnitTestCaseUsingDatabase {
     }
 
     public function tearDown() {
+        global $CFG;
         $CFG->stringfilters = $this->origcfgstringfilters;
         $CFG->filterall = $this->origcfgfilterall;