diff --git a/lib/classes/output/icon_system.php b/lib/classes/output/icon_system.php
index bad766f5632..08e88b54d2e 100644
--- a/lib/classes/output/icon_system.php
+++ b/lib/classes/output/icon_system.php
@@ -145,5 +145,12 @@ abstract class icon_system {
         }
         return false;
     }
+
+    /**
+     * Clears the instance cache, for use in unit tests
+     */
+    public static function reset_caches() {
+        self::$instance = null;
+    }
 }
 
diff --git a/lib/phpunit/classes/util.php b/lib/phpunit/classes/util.php
index 909024110e7..240777a869c 100644
--- a/lib/phpunit/classes/util.php
+++ b/lib/phpunit/classes/util.php
@@ -217,6 +217,7 @@ class phpunit_util extends testing_util {
         core_filetypes::reset_caches();
         \core_search\manager::clear_static();
         core_user::reset_caches();
+        \core\output\icon_system::reset_caches();
         if (class_exists('core_media_manager', false)) {
             core_media_manager::reset_caches();
         }