diff --git a/core/includes/module.inc b/core/includes/module.inc
index d9eab10..a772436 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -61,14 +61,29 @@ function module_load_all($bootstrap = FALSE) {
  *   list will persist until the next call with a new $fixed_list passed in.
  *   Primarily intended for internal use (e.g., in install.php and update.php).
  *   Use module_list_reset() to undo the $fixed_list override.
+ * @param bool $reset
+ *   (optional) Whether to reset/remove the $fixed_list.
  *
  * @return array
  *   An associative array whose keys and values are the names of the modules in
  *   the list.
+ *
+ * @see module_list_reset()
  */
-function module_list($type = 'module_enabled', array $fixed_list = NULL) {
-  // This static is only used for $fixed_list.
-  $module_list = &drupal_static(__FUNCTION__);
+function module_list($type = 'module_enabled', array $fixed_list = NULL, $reset = FALSE) {
+  // This static is only used for $fixed_list. It must not be a drupal_static(),
+  // since any call to drupal_static_reset() in unit tests would cause an
+  // attempt to retrieve the list of modules from the database (which does not
+  // exist).
+  static $module_list;
+
+  if ($reset) {
+    $module_list = NULL;
+    // Do nothing if no $type and no $fixed_list have been passed.
+    if (!isset($type) && !isset($fixed_list)) {
+      return;
+    }
+  }
 
   // The list that will be be returned. Separate from $module_list in order
   // to not duplicate the static cache of system_list().
@@ -94,7 +109,7 @@ function module_list($type = 'module_enabled', array $fixed_list = NULL) {
  * Subsequent calls to module_list() will no longer use a fixed list.
  */
 function module_list_reset() {
-  drupal_static_reset('module_list');
+  module_list(NULL, NULL, TRUE);
 }
 
 /**
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index d31cf1c..00eb07f 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -796,9 +796,8 @@ protected function tearDown() {
     // Reset all static variables.
     drupal_static_reset();
 
-    // Restore has_run state.
-    $has_run = &drupal_static('module_load_all');
-    $has_run = TRUE;
+    // Reset module list.
+    module_list_reset();
 
     // Restore original in-memory configuration.
     $conf = $this->originalConf;