diff --git a/gio/gdbusmessage.c b/gio/gdbusmessage.c
index eabe1715a..eb93c99a0 100644
--- a/gio/gdbusmessage.c
+++ b/gio/gdbusmessage.c
@@ -1195,8 +1195,12 @@ g_dbus_message_bytes_needed (guchar                *blob,
     }
   else if (blob[0] == 'B')
     {
-      /* TODO */
-      g_assert_not_reached ();
+      /* core header (12 bytes) + ARRAY of STRUCT of (BYTE,VARIANT) */
+      ret = 12 + 4 + GUINT32_FROM_BE (((guint32 *) blob)[3]);
+      /* round up so it's a multiple of 8 */
+      ret = 8 * ((ret + 7)/8);
+      /* finally add the body size */
+      ret += GUINT32_FROM_BE (((guint32 *) blob)[1]);
     }
   else
     {
diff --git a/gio/tests/gdbus-threading.c b/gio/tests/gdbus-threading.c
index 6a4e82cf8..24e070423 100644
--- a/gio/tests/gdbus-threading.c
+++ b/gio/tests/gdbus-threading.c
@@ -445,7 +445,7 @@ on_proxy_appeared (GDBusConnection *connection,
 
       /* elapsed_msec should be 4000 msec +/- change for overhead/inaccuracy */
       g_assert_cmpint (elapsed_msec, >=, 3950);
-      g_assert_cmpint (elapsed_msec,  <, 5000);
+      g_assert_cmpint (elapsed_msec,  <, 6000);
 
       g_print (" ");
     }