diff --git a/src/tests/README b/src/tests/README
index b7d9444ac..fb7d87eec 100644
--- a/src/tests/README
+++ b/src/tests/README
@@ -80,7 +80,10 @@ wait
 
 assert_stacking <client-id>/<window-id> <client-id>/<window-id> ...
   Assert that the list of client windows known to Mutter is as given and in
-  the given order, bottom to top.
+  the given order, bottom to top. The character '|' can be present in the
+  list of windows to indicate the guard window that separates hidden and
+  visible windows. If '|' isn't present, the guard window is asserted to
+  be below all client windows.
 
   This function also queries the X server stack and verifies that Mutter's
   expectation of the X server stack matches reality.
diff --git a/src/tests/stacking/override-redirect.metatest b/src/tests/stacking/override-redirect.metatest
index 96dde5b9d..b313d240b 100644
--- a/src/tests/stacking/override-redirect.metatest
+++ b/src/tests/stacking/override-redirect.metatest
@@ -12,7 +12,7 @@ assert_stacking 1/1 1/2
 
 lower 1/2
 wait
-assert_stacking 1/2 1/1
+assert_stacking 1/2 | 1/1
 
 raise 1/2
 wait
diff --git a/src/tests/test-runner.c b/src/tests/test-runner.c
index d5f723c3a..6996a87b3 100644
--- a/src/tests/test-runner.c
+++ b/src/tests/test-runner.c
@@ -610,6 +610,13 @@ test_case_assert_stacking (TestCase *test,
           else
             g_string_append_printf (stack_string, "(%s)", window->title);
         }
+      else if (windows[i] == display->screen->guard_window)
+        {
+          if (stack_string->len > 0)
+            g_string_append_c (stack_string, ' ');
+
+          g_string_append_c (stack_string, '|');
+        }
     }
 
   for (i = 0; i < n_expected_windows; i++)
@@ -620,6 +627,16 @@ test_case_assert_stacking (TestCase *test,
       g_string_append (expected_string, expected_windows[i]);
     }
 
+  /* Don't require '| ' as a prefix if there are no hidden windows - we
+   * remove the prefix from the actual string instead of adding it to the
+   * expected string for clarity of the error message
+   */
+  if (index (expected_string->str, '|') == NULL && stack_string->str[0] == '|')
+    {
+      g_string_erase (stack_string,
+                      0, stack_string->str[1] == ' ' ? 2 : 1);
+    }
+
   if (strcmp (expected_string->str, stack_string->str) != 0)
     {
       g_set_error (error, TEST_RUNNER_ERROR, TEST_RUNNER_ERROR_ASSERTION_FAILED,