diff --git a/dbus/dbus-connection.c b/dbus/dbus-connection.c
index 8c564deb..0694fe7f 100644
--- a/dbus/dbus-connection.c
+++ b/dbus/dbus-connection.c
@@ -1205,7 +1205,10 @@ _dbus_connection_do_iteration_unlocked (DBusConnection *connection,
                                         int             timeout_milliseconds)
 {
   _dbus_verbose ("start\n");
-  
+
+  /* All callers should have checked this */
+  _dbus_assert (timeout_milliseconds >= -1);
+
   HAVE_LOCK_CHECK (connection);
   
   if (connection->n_outgoing == 0)
@@ -3694,6 +3697,9 @@ _dbus_connection_read_write_dispatch (DBusConnection *connection,
   DBusDispatchStatus dstatus;
   dbus_bool_t progress_possible;
 
+  /* All callers should have checked this */
+  _dbus_assert (timeout_milliseconds >= -1);
+
   /* Need to grab a ref here in case we're a private connection and
    * the user drops the last ref in a handler we call; see bug 
    * https://bugs.freedesktop.org/show_bug.cgi?id=15635
diff --git a/dbus/dbus-transport.c b/dbus/dbus-transport.c
index dd31a82b..32f47a87 100644
--- a/dbus/dbus-transport.c
+++ b/dbus/dbus-transport.c
@@ -1003,6 +1003,9 @@ _dbus_transport_do_iteration (DBusTransport  *transport,
                               unsigned int    flags,
                               int             timeout_milliseconds)
 {
+  /* All callers should have checked this */
+  _dbus_assert (timeout_milliseconds >= -1);
+
   _dbus_assert (transport->vtable->do_iteration != NULL);
 
   _dbus_verbose ("Transport iteration flags 0x%x timeout %d connected = %d\n",