diff --git a/tabbox/tabboxhandler.cpp b/tabbox/tabboxhandler.cpp
index 242dcb3f15..66d2acb1af 100644
--- a/tabbox/tabboxhandler.cpp
+++ b/tabbox/tabboxhandler.cpp
@@ -124,7 +124,11 @@ void TabBoxHandlerPrivate::updateOutline()
         q->hideOutline();
         return;
     }
-    TabBoxClient* c = static_cast< TabBoxClient* >(m_clientModel->data(index, ClientModel::ClientRole).value<void *>());
+    const QVariant client = m_clientModel->data(index, ClientModel::ClientRole);
+    if (!client.isValid()) {
+        return;
+    }
+    TabBoxClient* c = static_cast< TabBoxClient* >(client.value<void *>());
     q->showOutline(QRect(c->x(), c->y(), c->width(), c->height()));
 }
 
diff --git a/tabbox/tests/CMakeLists.txt b/tabbox/tests/CMakeLists.txt
index 3478e99265..b090785df7 100644
--- a/tabbox/tests/CMakeLists.txt
+++ b/tabbox/tests/CMakeLists.txt
@@ -15,3 +15,21 @@ set( testTabBoxClientModel_SRCS
 kde4_add_unit_test( testTabBoxClientModel TESTNAME testTabBoxClientModel ${testTabBoxClientModel_SRCS} )
 
 target_link_libraries( testTabBoxClientModel ${KDE4_KDEUI_LIBS} ${QT_QTCORE_LIBRARY} ${QT_QTGUI_LIBRARY} ${QT_QTDECLARATIVE_LIBRARY} ${QT_QTTEST_LIBRARY} ${X11_LIBRARIES} )
+
+########################################################
+# Test TabBox::TabBoxHandler
+########################################################
+set( testTabBoxHandler_SRCS
+     ../clientmodel.cpp
+     ../desktopmodel.cpp
+     ../tabboxconfig.cpp
+     ../tabboxhandler.cpp
+     test_tabbox_handler.cpp
+     mock_declarative.cpp
+     mock_tabboxhandler.cpp
+     mock_tabboxclient.cpp
+)
+
+kde4_add_unit_test( testTabBoxHandler TESTNAME testTabBoxHandler ${testTabBoxHandler_SRCS} )
+
+target_link_libraries( testTabBoxHandler ${KDE4_KDEUI_LIBS} ${QT_QTCORE_LIBRARY} ${QT_QTGUI_LIBRARY} ${QT_QTDECLARATIVE_LIBRARY} ${QT_QTTEST_LIBRARY} ${X11_LIBRARIES} )
diff --git a/tabbox/tests/test_tabbox_handler.cpp b/tabbox/tests/test_tabbox_handler.cpp
new file mode 100644
index 0000000000..5c081774a7
--- /dev/null
+++ b/tabbox/tests/test_tabbox_handler.cpp
@@ -0,0 +1,55 @@
+/********************************************************************
+KWin - the KDE window manager
+This file is part of the KDE project.
+
+Copyright (C) 2012 Martin Gräßlin <mgraesslin@kde.org>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*********************************************************************/
+#include "mock_tabboxhandler.h"
+#include "clientmodel.h"
+#include <QtTest/QtTest>
+
+using namespace KWin;
+
+class TestTabBoxHandler : public QObject
+{
+    Q_OBJECT
+private slots:
+    /**
+     * Test to verify that update outline does not crash
+     * if the ModelIndex for which the outline should be
+     * shown is not valid. That is accessing the Pointer
+     * to the Client returns an invalid QVariant.
+     * BUG: 304620
+     **/
+    void testDontCrashUpdateOutlineNullClient();
+};
+
+void TestTabBoxHandler::testDontCrashUpdateOutlineNullClient()
+{
+    MockTabBoxHandler tabboxhandler;
+    TabBox::TabBoxConfig config;
+    config.setTabBoxMode(TabBox::TabBoxConfig::ClientTabBox);
+    config.setShowOutline(true);
+    config.setShowTabBox(false);
+    config.setHighlightWindows(false);
+    tabboxhandler.setConfig(config);
+    // now show the tabbox which will attempt to show the outline
+    tabboxhandler.show();
+}
+
+QTEST_MAIN(TestTabBoxHandler)
+
+#include "test_tabbox_handler.moc"