diff --git a/tabbox/CMakeLists.txt b/tabbox/CMakeLists.txt
index e222b6a63f..2ef58101fa 100644
--- a/tabbox/CMakeLists.txt
+++ b/tabbox/CMakeLists.txt
@@ -1,4 +1,5 @@
 add_subdirectory( qml )
+add_subdirectory( tests )
 
 # Install the KWin/WindowSwitcher service type
 install( FILES kwinwindowswitcher.desktop DESTINATION ${SERVICETYPES_INSTALL_DIR} )
diff --git a/tabbox/clientmodel.cpp b/tabbox/clientmodel.cpp
index 06a0c1a869..b78c1f5ce4 100644
--- a/tabbox/clientmodel.cpp
+++ b/tabbox/clientmodel.cpp
@@ -93,6 +93,9 @@ QString ClientModel::longestCaption() const
     QString caption;
     foreach (QWeakPointer<TabBoxClient> clientPointer, m_clientList) {
         QSharedPointer<TabBoxClient> client = clientPointer.toStrongRef();
+        if (!client) {
+            continue;
+        }
         if (client->caption().size() > caption.size()) {
             caption = client->caption();
         }
diff --git a/tabbox/tests/CMakeLists.txt b/tabbox/tests/CMakeLists.txt
new file mode 100644
index 0000000000..f8377ac929
--- /dev/null
+++ b/tabbox/tests/CMakeLists.txt
@@ -0,0 +1,17 @@
+########################################################
+# Test TabBox::ClientModel
+########################################################
+set( testTabBoxClientModel_SRCS
+     ../clientmodel.cpp
+     ../desktopmodel.cpp
+     ../tabboxconfig.cpp
+     ../tabboxhandler.cpp
+     test_tabbox_clientmodel.cpp
+     mock_declarative.cpp
+     mock_tabboxhandler.cpp
+     mock_tabboxclient.cpp
+)
+
+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} )
diff --git a/tabbox/tests/mock_declarative.cpp b/tabbox/tests/mock_declarative.cpp
new file mode 100644
index 0000000000..67ac56a9be
--- /dev/null
+++ b/tabbox/tests/mock_declarative.cpp
@@ -0,0 +1,94 @@
+/********************************************************************
+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 "declarative.h"
+
+namespace KWin
+{
+namespace TabBox
+{
+DeclarativeView::DeclarativeView(QAbstractItemModel *model, TabBoxConfig::TabBoxMode mode, QWidget *parent)
+    : QDeclarativeView(parent)
+{
+    Q_UNUSED(model)
+    Q_UNUSED(mode)
+}
+
+void DeclarativeView::currentIndexChanged(int row)
+{
+    Q_UNUSED(row)
+}
+
+void DeclarativeView::updateQmlSource(bool force)
+{
+    Q_UNUSED(force)
+}
+
+void DeclarativeView::setCurrentIndex(const QModelIndex &index, bool disableAnimation)
+{
+    Q_UNUSED(index)
+    Q_UNUSED(disableAnimation)
+}
+
+bool DeclarativeView::sendKeyEvent(QKeyEvent *e)
+{
+    Q_UNUSED(e)
+    return false;
+}
+
+void DeclarativeView::slotEmbeddedChanged(bool enabled)
+{
+    Q_UNUSED(enabled)
+}
+
+void DeclarativeView::slotUpdateGeometry()
+{
+}
+
+void DeclarativeView::slotWindowChanged(WId wId, unsigned int properties)
+{
+    Q_UNUSED(wId)
+    Q_UNUSED(properties)
+}
+
+void DeclarativeView::showEvent(QShowEvent *event)
+{
+    Q_UNUSED(event)
+}
+
+void DeclarativeView::resizeEvent(QResizeEvent *event)
+{
+    Q_UNUSED(event)
+}
+
+void DeclarativeView::hideEvent(QHideEvent *event)
+{
+    Q_UNUSED(event)
+}
+
+bool DeclarativeView::x11Event(XEvent *e)
+{
+    Q_UNUSED(e)
+    return false;
+}
+
+} // namespace Tabbox
+} // namespace KWin
+
+#include "../declarative.moc"
diff --git a/tabbox/tests/mock_tabboxclient.cpp b/tabbox/tests/mock_tabboxclient.cpp
new file mode 100644
index 0000000000..5b5f09ec01
--- /dev/null
+++ b/tabbox/tests/mock_tabboxclient.cpp
@@ -0,0 +1,38 @@
+/********************************************************************
+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_tabboxclient.h"
+#include "mock_tabboxhandler.h"
+
+namespace KWin
+{
+
+MockTabBoxClient::MockTabBoxClient(QString caption, WId id)
+    : TabBoxClient()
+    , m_caption(caption)
+    , m_wId(id)
+{
+}
+
+void MockTabBoxClient::close()
+{
+    static_cast<MockTabBoxHandler*>(TabBox::tabBox)->closeWindow(this);
+}
+
+} // namespace KWin
diff --git a/tabbox/tests/mock_tabboxclient.h b/tabbox/tests/mock_tabboxclient.h
new file mode 100644
index 0000000000..ec438e603e
--- /dev/null
+++ b/tabbox/tests/mock_tabboxclient.h
@@ -0,0 +1,67 @@
+/********************************************************************
+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/>.
+*********************************************************************/
+#ifndef KWIN_MOCK_TABBOX_CLIENT_H
+#define KWIN_MOCK_TABBOX_CLIENT_H
+
+#include "../tabboxhandler.h"
+namespace KWin
+{
+class MockTabBoxClient : public TabBox::TabBoxClient
+{
+public:
+    explicit MockTabBoxClient(QString caption, WId id);
+    virtual bool isMinimized() const {
+        return false;
+    }
+    virtual QString caption() const {
+        return m_caption;
+    }
+    virtual void close();
+    virtual int height() const {
+        return 100;
+    }
+    virtual QPixmap icon(const QSize &size = QSize(32, 32)) const {
+        return QPixmap(size);
+    }
+    virtual bool isCloseable() const {
+        return true;
+    }
+    virtual bool isFirstInTabBox() const {
+        return false;
+    }
+    virtual int width() const {
+        return 100;
+    }
+    virtual WId window() const {
+        return m_wId;
+    }
+    virtual int x() const {
+        return 0;
+    }
+    virtual int y() const {
+        return 0;
+    }
+
+private:
+    QString m_caption;
+    WId m_wId;
+};
+} // namespace KWin
+#endif
diff --git a/tabbox/tests/mock_tabboxhandler.cpp b/tabbox/tests/mock_tabboxhandler.cpp
new file mode 100644
index 0000000000..1d27dc752b
--- /dev/null
+++ b/tabbox/tests/mock_tabboxhandler.cpp
@@ -0,0 +1,99 @@
+/********************************************************************
+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 "mock_tabboxclient.h"
+
+namespace KWin
+{
+
+MockTabBoxHandler::MockTabBoxHandler()
+    : TabBoxHandler()
+{
+}
+
+MockTabBoxHandler::~MockTabBoxHandler()
+{
+}
+
+void MockTabBoxHandler::grabbedKeyEvent(QKeyEvent *event) const
+{
+    Q_UNUSED(event)
+}
+
+QWeakPointer< TabBox::TabBoxClient > MockTabBoxHandler::activeClient() const
+{
+    if (!m_windows.isEmpty()) {
+        return QWeakPointer< TabBox::TabBoxClient >(m_windows.first());
+    }
+    return QWeakPointer<TabBox::TabBoxClient>();
+}
+
+QWeakPointer< TabBox::TabBoxClient > MockTabBoxHandler::clientToAddToList(TabBox::TabBoxClient *client, int desktop) const
+{
+    Q_UNUSED(desktop)
+    QList< QSharedPointer< TabBox::TabBoxClient > >::const_iterator it = m_windows.constBegin();
+    for (; it != m_windows.constEnd(); ++it) {
+        if ((*it).data() == client) {
+            return QWeakPointer< TabBox::TabBoxClient >(*it);
+        }
+    }
+    return QWeakPointer< TabBox::TabBoxClient >();
+}
+
+QWeakPointer< TabBox::TabBoxClient > MockTabBoxHandler::nextClientFocusChain(TabBox::TabBoxClient *client) const
+{
+    if (!client) {
+        if (!m_windows.isEmpty()) {
+            return QWeakPointer< TabBox::TabBoxClient >(m_windows.first());
+        }
+    }
+    QList< QSharedPointer< TabBox::TabBoxClient > >::const_iterator it = m_windows.constBegin();
+    for (; it != m_windows.constEnd(); ++it) {
+        if ((*it).data() == client) {
+            ++it;
+            if (it == m_windows.constEnd()) {
+                return QWeakPointer< TabBox::TabBoxClient >(m_windows.first());
+            } else {
+                return QWeakPointer< TabBox::TabBoxClient >(*it);
+            }
+        }
+    }
+    return QWeakPointer< TabBox::TabBoxClient >();
+}
+
+QWeakPointer< TabBox::TabBoxClient > MockTabBoxHandler::createMockWindow(const QString &caption, WId id)
+{
+    QSharedPointer< TabBox::TabBoxClient > client(new MockTabBoxClient(caption, id));
+    m_windows.append(client);
+    return QWeakPointer< TabBox::TabBoxClient >(client);
+}
+
+void MockTabBoxHandler::closeWindow(TabBox::TabBoxClient *client)
+{
+    QList< QSharedPointer< TabBox::TabBoxClient > >::iterator it = m_windows.begin();
+    for (; it != m_windows.end(); ++it) {
+        if ((*it).data() == client) {
+            m_windows.erase(it);
+            return;
+        }
+    }
+}
+
+} // namespace KWin
diff --git a/tabbox/tests/mock_tabboxhandler.h b/tabbox/tests/mock_tabboxhandler.h
new file mode 100644
index 0000000000..f13a820aac
--- /dev/null
+++ b/tabbox/tests/mock_tabboxhandler.h
@@ -0,0 +1,93 @@
+/********************************************************************
+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/>.
+*********************************************************************/
+#ifndef KWIN_MOCK_TABBOX_HANDLER_H
+#define KWIN_MOCK_TABBOX_HANDLER_H
+
+#include "../tabboxhandler.h"
+namespace KWin
+{
+class MockTabBoxHandler : public TabBox::TabBoxHandler
+{
+    Q_OBJECT
+public:
+    MockTabBoxHandler();
+    virtual ~MockTabBoxHandler();
+    virtual void activateAndClose() {
+    }
+    virtual QWeakPointer< TabBox::TabBoxClient > activeClient() const;
+    virtual int activeScreen() const {
+        return 0;
+    }
+    virtual QWeakPointer< TabBox::TabBoxClient > clientToAddToList(TabBox::TabBoxClient *client, int desktop) const;
+    virtual int currentDesktop() const {
+        return 1;
+    }
+    virtual QWeakPointer< TabBox::TabBoxClient > desktopClient() const {
+        return QWeakPointer<TabBox::TabBoxClient>();
+    }
+    virtual QString desktopName(int desktop) const {
+        Q_UNUSED(desktop)
+        return "desktop 1";
+    }
+    virtual QString desktopName(TabBox::TabBoxClient *client) const {
+        Q_UNUSED(client)
+        return "desktop";
+    }
+    virtual void elevateClient(TabBox::TabBoxClient *c, WId tabbox, bool elevate) const {
+        Q_UNUSED(c)
+        Q_UNUSED(tabbox)
+        Q_UNUSED(elevate)
+    }
+    virtual void hideOutline() {
+    }
+    virtual QWeakPointer< TabBox::TabBoxClient > nextClientFocusChain(TabBox::TabBoxClient *client) const;
+    virtual int nextDesktopFocusChain(int desktop) const {
+        Q_UNUSED(desktop)
+        return 1;
+    }
+    virtual int numberOfDesktops() const {
+        return 1;
+    }
+    virtual QVector< Window > outlineWindowIds() const {
+        return QVector<Window>();
+    }
+    virtual void raiseClient(TabBox::TabBoxClient *c) const {
+        Q_UNUSED(c)
+    }
+    virtual void restack(TabBox::TabBoxClient *c, TabBox::TabBoxClient *under) {
+        Q_UNUSED(c)
+        Q_UNUSED(under)
+    }
+    virtual void showOutline(const QRect &outline) {
+        Q_UNUSED(outline)
+    }
+    virtual TabBox::TabBoxClientList stackingOrder() const {
+        return TabBox::TabBoxClientList();
+    }
+    virtual void grabbedKeyEvent(QKeyEvent *event) const;
+
+    // mock methods
+    QWeakPointer<TabBox::TabBoxClient> createMockWindow(const QString &caption, WId id);
+    void closeWindow(TabBox::TabBoxClient *client);
+private:
+    QList< QSharedPointer<TabBox::TabBoxClient> > m_windows;
+};
+} // namespace KWin
+#endif
diff --git a/tabbox/tests/test_tabbox_clientmodel.cpp b/tabbox/tests/test_tabbox_clientmodel.cpp
new file mode 100644
index 0000000000..cd2c963629
--- /dev/null
+++ b/tabbox/tests/test_tabbox_clientmodel.cpp
@@ -0,0 +1,47 @@
+/********************************************************************
+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 "test_tabbox_clientmodel.h"
+#include "mock_tabboxhandler.h"
+#include "clientmodel.h"
+
+#include <QtTest/QtTest>
+using namespace KWin;
+
+void TestTabBoxClientModel::testLongestCaptionWithNullClient()
+{
+    MockTabBoxHandler tabboxhandler;
+    TabBox::ClientModel *clientModel = new TabBox::ClientModel(&tabboxhandler);
+    clientModel->createClientList();
+    QCOMPARE(clientModel->longestCaption(), QString());
+    // add a window to the mock
+    tabboxhandler.createMockWindow(QString("test"), 1);
+    clientModel->createClientList();
+    QCOMPARE(clientModel->longestCaption(), QString("test"));
+    // delete the one client in the list
+    QModelIndex index = clientModel->index(0, 0);
+    QVERIFY(index.isValid());
+    TabBox::TabBoxClient *client = static_cast<TabBox::TabBoxClient *>(clientModel->data(index, TabBox::ClientModel::ClientRole).value<void*>());
+    client->close();
+    // internal model of ClientModel now contains a deleted pointer
+    // longestCaption should behave just as if the window were not in the list
+    QCOMPARE(clientModel->longestCaption(), QString());
+}
+
+QTEST_MAIN(TestTabBoxClientModel)
diff --git a/tabbox/tests/test_tabbox_clientmodel.h b/tabbox/tests/test_tabbox_clientmodel.h
new file mode 100644
index 0000000000..1bdf0d4aa4
--- /dev/null
+++ b/tabbox/tests/test_tabbox_clientmodel.h
@@ -0,0 +1,38 @@
+/********************************************************************
+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/>.
+*********************************************************************/
+#ifndef TEST_TABBOX_CLIENT_MODEL_H
+#define TEST_TABBOX_CLIENT_MODEL_H
+#include <QObject>
+
+class TestTabBoxClientModel : public QObject
+{
+    Q_OBJECT
+private slots:
+    /**
+     * Tests that calculating the longest caption does not
+     * crash in case the internal m_clientList contains a weak
+     * pointer to a deleted TabBoxClient.
+     *
+     * See bug #303840
+     **/
+    void testLongestCaptionWithNullClient();
+};
+
+#endif