diff --git a/src/core/window-private.h b/src/core/window-private.h
index ec2cfd762..8583eefca 100644
--- a/src/core/window-private.h
+++ b/src/core/window-private.h
@@ -189,7 +189,6 @@ struct _MetaWindow
   char *res_class;
   char *res_name;
   char *role;
-  char *sm_client_id;
   char *wm_client_machine;
 
   char *startup_id;
diff --git a/src/core/window.c b/src/core/window.c
index a3e5a7685..be5f45859 100644
--- a/src/core/window.c
+++ b/src/core/window.c
@@ -324,7 +324,6 @@ meta_window_finalize (GObject *object)
   if (window->cgroup_path)
     g_object_unref (window->cgroup_path);
 
-  g_free (window->sm_client_id);
   g_free (window->wm_client_machine);
   g_free (window->startup_id);
   g_free (window->role);
@@ -1098,7 +1097,6 @@ meta_window_constructed (GObject *object)
   window->res_class = NULL;
   window->res_name = NULL;
   window->role = NULL;
-  window->sm_client_id = NULL;
   window->wm_client_machine = NULL;
   window->is_remote = FALSE;
   window->startup_id = NULL;
diff --git a/src/x11/session.c b/src/x11/session.c
index c40a77d9e..8da4cc089 100644
--- a/src/x11/session.c
+++ b/src/x11/session.c
@@ -32,6 +32,7 @@
 #include "core/util-private.h"
 #include "meta/meta-context.h"
 #include "x11/meta-x11-display-private.h"
+#include "x11/window-x11-private.h"
 
 #ifndef HAVE_SM
 void
@@ -959,10 +960,12 @@ save_state (MetaContext *context)
   while (tmp != NULL)
     {
       MetaWindow *window;
+      MetaWindowX11Private *priv;
 
       window = tmp->data;
+      priv = meta_window_x11_get_private (META_WINDOW_X11 (window));
 
-      if (window->sm_client_id)
+      if (priv->sm_client_id)
         {
           char *sm_client_id;
           char *res_class;
@@ -975,7 +978,7 @@ save_state (MetaContext *context)
            * in practice they are always ascii though.)
            */
 
-          sm_client_id = encode_text_as_utf8_markup (window->sm_client_id);
+          sm_client_id = encode_text_as_utf8_markup (priv->sm_client_id);
           res_class = window->res_class ?
             encode_text_as_utf8_markup (window->res_class) : NULL;
           res_name = window->res_name ?
@@ -988,7 +991,7 @@ save_state (MetaContext *context)
             title = NULL;
 
           meta_topic (META_DEBUG_SM, "Saving session managed window %s, client ID '%s'",
-                      window->desc, window->sm_client_id);
+                      window->desc, priv->sm_client_id);
 
           fprintf (outfile,
                    "  <window id=\"%s\" class=\"%s\" name=\"%s\" title=\"%s\" role=\"%s\" type=\"%s\" stacking=\"%d\">\n",
@@ -1566,10 +1569,12 @@ get_possible_matches (MetaWindow *window)
   GSList *retval;
   GSList *tmp;
   gboolean ignore_client_id;
+  MetaWindowX11Private *priv;
 
   retval = NULL;
 
   ignore_client_id = g_getenv ("MUTTER_DEBUG_SM") != NULL;
+  priv = meta_window_x11_get_private (META_WINDOW_X11 (window));
 
   tmp = window_info_list;
   while (tmp != NULL)
@@ -1579,7 +1584,7 @@ get_possible_matches (MetaWindow *window)
       info = tmp->data;
 
       if ((ignore_client_id ||
-           both_null_or_matching (info->id, window->sm_client_id)) &&
+           both_null_or_matching (info->id, priv->sm_client_id)) &&
           both_null_or_matching (info->res_class, window->res_class) &&
           both_null_or_matching (info->res_name, window->res_name) &&
           both_null_or_matching (info->role, window->role))
@@ -1596,10 +1601,10 @@ get_possible_matches (MetaWindow *window)
         {
           if (meta_is_verbose ())
             {
-              if (!both_null_or_matching (info->id, window->sm_client_id))
+              if (!both_null_or_matching (info->id, priv->sm_client_id))
                 meta_topic (META_DEBUG_SM, "Window %s has SM client ID %s, saved state has %s, no match",
                             window->desc,
-                            window->sm_client_id ? window->sm_client_id : "(none)",
+                            priv->sm_client_id ? priv->sm_client_id : "(none)",
                             info->id ? info->id : "(none)");
               else if (!both_null_or_matching (info->res_class, window->res_class))
                 meta_topic (META_DEBUG_SM, "Window %s has class %s doesn't match saved class %s, no match",
@@ -1677,13 +1682,15 @@ meta_window_lookup_saved_state (MetaWindow *window)
 {
   GSList *possibles;
   const MetaWindowSessionInfo *info;
+  MetaWindowX11Private *priv =
+    meta_window_x11_get_private (META_WINDOW_X11 (window));
 
   /* Window is not session managed.
    * I haven't yet figured out how to deal with these
    * in a way that doesn't cause broken side effects in
    * situations other than on session restore.
    */
-  if (window->sm_client_id == NULL)
+  if (priv->sm_client_id == NULL)
     {
       meta_topic (META_DEBUG_SM,
                   "Window %s is not session managed, not checking for saved state",
diff --git a/src/x11/window-x11-private.h b/src/x11/window-x11-private.h
index 8b3ac2a4e..993a90465 100644
--- a/src/x11/window-x11-private.h
+++ b/src/x11/window-x11-private.h
@@ -95,6 +95,8 @@ struct _MetaWindowX11Private
   /* Used by keybindings.c */
   gboolean keys_grabbed;     /* normal keybindings grabbed */
   gboolean grab_on_frame;    /* grabs are on the frame */
+
+  char *sm_client_id;
 };
 
 MetaWindowX11Private * meta_window_x11_get_private (MetaWindowX11 *window_x11);
diff --git a/src/x11/window-x11.c b/src/x11/window-x11.c
index 18ed0a5f4..0cdc032b2 100644
--- a/src/x11/window-x11.c
+++ b/src/x11/window-x11.c
@@ -188,7 +188,7 @@ update_sm_hints (MetaWindow *window)
   Window leader;
 
   priv->xclient_leader = None;
-  window->sm_client_id = NULL;
+  priv->sm_client_id = NULL;
 
   /* If not on the current window, we can get the client
    * leader from transient parents. If we find a client
@@ -211,7 +211,7 @@ update_sm_hints (MetaWindow *window)
 
       meta_prop_get_latin1_string (window->display->x11_display, leader,
                                    window->display->x11_display->atom_SM_CLIENT_ID,
-                                   &window->sm_client_id);
+                                   &priv->sm_client_id);
     }
   else
     {
@@ -225,9 +225,9 @@ update_sm_hints (MetaWindow *window)
           meta_prop_get_latin1_string (window->display->x11_display,
                                        priv->xwindow,
                                        window->display->x11_display->atom_SM_CLIENT_ID,
-                                       &window->sm_client_id);
+                                       &priv->sm_client_id);
 
-          if (window->sm_client_id)
+          if (priv->sm_client_id)
             meta_warning ("Window %s sets SM_CLIENT_ID on itself, instead of on the WM_CLIENT_LEADER window as specified in the ICCCM.",
                           window->desc);
         }
@@ -235,7 +235,7 @@ update_sm_hints (MetaWindow *window)
 
   meta_verbose ("Window %s client leader: 0x%lx SM_CLIENT_ID: '%s'",
                 window->desc, priv->xclient_leader,
-                window->sm_client_id ? window->sm_client_id : "none");
+                priv->sm_client_id ? priv->sm_client_id : "none");
 }
 
 static void
@@ -2143,6 +2143,17 @@ meta_window_x11_set_property (GObject      *object,
     }
 }
 
+static void
+meta_window_x11_finalize (GObject *object)
+{
+  MetaWindowX11 *win = META_WINDOW_X11 (object);
+  MetaWindowX11Private *priv = meta_window_x11_get_instance_private (win);
+
+  g_clear_pointer (&priv->sm_client_id, g_free);
+
+  G_OBJECT_CLASS (meta_window_x11_parent_class)->finalize (object);
+}
+
 static void
 meta_window_x11_class_init (MetaWindowX11Class *klass)
 {
@@ -2152,6 +2163,7 @@ meta_window_x11_class_init (MetaWindowX11Class *klass)
   object_class->get_property = meta_window_x11_get_property;
   object_class->set_property = meta_window_x11_set_property;
   object_class->constructed = meta_window_x11_constructed;
+  object_class->finalize = meta_window_x11_finalize;
 
   window_class->manage = meta_window_x11_manage;
   window_class->unmanage = meta_window_x11_unmanage;