Erro ao abrir jInternalFrame

Exception in thread "AWT-EventQueue-0" java.lang.IllegalArgumentException: adding a window to a container
        at java.awt.Container.checkNotAWindow(Container.java:431)
        at java.awt.Container.addImpl(Container.java:1039)
        at javax.swing.JLayeredPane.addImpl(JLayeredPane.java:212)
        at javax.swing.JDesktopPane.addImpl(JDesktopPane.java:470)
        at java.awt.Container.add(Container.java:365)
        at sistema.SistemaHome.visualizarClientes(SistemaHome.java:442)
        at sistema.SistemaHome.BT_ClientesActionPerformed(SistemaHome.java:348)
        at sistema.SistemaHome.access$1400(SistemaHome.java:25)
        at sistema.SistemaHome$10.actionPerformed(SistemaHome.java:192)
        at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
        at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
        at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
        at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
        at java.awt.Component.processMouseEvent(Component.java:6267)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
        at java.awt.Component.processEvent(Component.java:6032)
        at java.awt.Container.processEvent(Container.java:2041)
        at java.awt.Component.dispatchEventImpl(Component.java:4630)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
        at java.awt.Container.dispatchEventImpl(Container.java:2085)
        at java.awt.Window.dispatchEventImpl(Window.java:2478)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
CONSTRUÍDO COM SUCESSO (tempo total: 55 segundos)

private void visualizarClientes() throws SQLException {
        if(fClientes == null){
            fClientes = new FrameClientes();
        }
    jDesktopPane1.add(fClientes);
        fClientes.setVisible(true);
        int lDesk = jDesktopPane1.getWidth();
        int aDesk = jDesktopPane1.getHeight();
        int lwFrame = fClientes.getWidth();
        int lhFrame = fClientes.getHeight();
        fClientes.setLocation(lDesk / 2 - lwFrame /2, aDesk /2 - lhFrame /2);

    }