diff --git a/src/core/screen.c b/src/core/screen.c
index e8e8c56e0..c68ee10ff 100644
--- a/src/core/screen.c
+++ b/src/core/screen.c
@@ -389,6 +389,8 @@ int
 meta_screen_monitor_index_to_xinerama_index (MetaScreen *screen,
                                              int         index)
 {
+  g_return_val_if_fail (index >= 0 && index < screen->n_monitor_infos, -1);
+
   meta_screen_ensure_xinerama_indices (screen);
 
   return screen->monitor_infos[index].xinerama_index;
diff --git a/src/core/window-x11.c b/src/core/window-x11.c
index d984a59c3..150390a65 100644
--- a/src/core/window-x11.c
+++ b/src/core/window-x11.c
@@ -127,23 +127,35 @@ meta_window_x11_set_net_wm_state (MetaWindow *window)
 
   if (window->fullscreen)
     {
-      data[0] = meta_screen_monitor_index_to_xinerama_index (window->screen,
-                                                             window->fullscreen_monitors[0]);
-      data[1] = meta_screen_monitor_index_to_xinerama_index (window->screen,
-                                                             window->fullscreen_monitors[1]);
-      data[2] = meta_screen_monitor_index_to_xinerama_index (window->screen,
-                                                             window->fullscreen_monitors[2]);
-      data[3] = meta_screen_monitor_index_to_xinerama_index (window->screen,
-                                                             window->fullscreen_monitors[3]);
+      if (window->fullscreen_monitors[0] >= 0)
+        {
+          data[0] = meta_screen_monitor_index_to_xinerama_index (window->screen,
+                                                                 window->fullscreen_monitors[0]);
+          data[1] = meta_screen_monitor_index_to_xinerama_index (window->screen,
+                                                                 window->fullscreen_monitors[1]);
+          data[2] = meta_screen_monitor_index_to_xinerama_index (window->screen,
+                                                                 window->fullscreen_monitors[2]);
+          data[3] = meta_screen_monitor_index_to_xinerama_index (window->screen,
+                                                                 window->fullscreen_monitors[3]);
 
-      meta_verbose ("Setting _NET_WM_FULLSCREEN_MONITORS\n");
-      meta_error_trap_push (window->display);
-      XChangeProperty (window->display->xdisplay,
-                       window->xwindow,
-                       window->display->atom__NET_WM_FULLSCREEN_MONITORS,
-                       XA_CARDINAL, 32, PropModeReplace,
-                       (guchar*) data, 4);
-      meta_error_trap_pop (window->display);
+          meta_verbose ("Setting _NET_WM_FULLSCREEN_MONITORS\n");
+          meta_error_trap_push (window->display);
+          XChangeProperty (window->display->xdisplay,
+                           window->xwindow,
+                           window->display->atom__NET_WM_FULLSCREEN_MONITORS,
+                           XA_CARDINAL, 32, PropModeReplace,
+                           (guchar*) data, 4);
+          meta_error_trap_pop (window->display);
+        }
+      else
+        {
+          meta_verbose ("Clearing _NET_WM_FULLSCREEN_MONITORS\n");
+          meta_error_trap_push (window->display);
+          XDeleteProperty (window->display->xdisplay,
+                           window->xwindow,
+                           window->display->atom__NET_WM_FULLSCREEN_MONITORS);
+          meta_error_trap_pop (window->display);
+        }
     }
 }