diff --git a/src/kassistantdialog.h b/src/kassistantdialog.h
--- a/src/kassistantdialog.h
+++ b/src/kassistantdialog.h
@@ -54,7 +54,7 @@
      * @flags the window flags to give to the assistant dialog. The
      * default of zero is usually what you want.
      */
-    explicit KAssistantDialog(QWidget *parent = nullptr, Qt::WindowFlags flags = nullptr);
+    explicit KAssistantDialog(QWidget *parent = nullptr, Qt::WindowFlags flags = Qt::WindowFlags());
     virtual ~KAssistantDialog();
 
     /**
@@ -140,7 +140,7 @@
      *
      * Calls the KPageDialog(KPageWidget *widget, QWidget *parent, Qt::WindowFlags flags) constructor
      */
-    explicit KAssistantDialog(KPageWidget *widget, QWidget *parent = nullptr, Qt::WindowFlags flags = nullptr);
+    explicit KAssistantDialog(KPageWidget *widget, QWidget *parent = nullptr, Qt::WindowFlags flags = Qt::WindowFlags());
 
     void showEvent(QShowEvent *event) Q_DECL_OVERRIDE;
 
diff --git a/src/keditlistwidget.cpp b/src/keditlistwidget.cpp
--- a/src/keditlistwidget.cpp
+++ b/src/keditlistwidget.cpp
@@ -90,7 +90,7 @@
 
     setEditor(lineEdit, representationWidget);
 
-    buttons = nullptr;
+    buttons = KEditListWidget::Buttons();
     q->setButtons(newButtons);
 
     q->connect(listView->selectionModel(), &QItemSelectionModel::selectionChanged,
diff --git a/src/kfontrequester.cpp b/src/kfontrequester.cpp
--- a/src/kfontrequester.cpp
+++ b/src/kfontrequester.cpp
@@ -184,7 +184,7 @@
 
 void KFontRequester::KFontRequesterPrivate::_k_buttonClicked()
 {
-    QFontDialog::FontDialogOptions flags = nullptr;
+    QFontDialog::FontDialogOptions flags;
     if (m_onlyFixed) {
         flags = QFontDialog::MonospacedFonts;
     }
diff --git a/src/kpagedialog.h b/src/kpagedialog.h
--- a/src/kpagedialog.h
+++ b/src/kpagedialog.h
@@ -108,7 +108,7 @@
     /**
      * Creates a new page dialog.
      */
-    explicit KPageDialog(QWidget *parent = nullptr, Qt::WindowFlags flags = nullptr);
+    explicit KPageDialog(QWidget *parent = nullptr, Qt::WindowFlags flags = Qt::WindowFlags());
 
     /**
      * Destroys the page dialog.
@@ -235,8 +235,8 @@
      * \param widget The KPageWidget object will be reparented to this object, so you can create
      * it without parent and you are not allowed to delete it.
      */
-    KPageDialog(KPageWidget *widget, QWidget *parent, Qt::WindowFlags flags = nullptr);
-    KPageDialog(KPageDialogPrivate &dd, KPageWidget *widget, QWidget *parent, Qt::WindowFlags flags = nullptr);
+    KPageDialog(KPageWidget *widget, QWidget *parent, Qt::WindowFlags flags = Qt::WindowFlags());
+    KPageDialog(KPageDialogPrivate &dd, KPageWidget *widget, QWidget *parent, Qt::WindowFlags flags = Qt::WindowFlags());
 
     /**
      * Returns the page widget of the dialog or a null pointer if no page widget is set.
diff --git a/src/kpagewidgetmodel.cpp b/src/kpagewidgetmodel.cpp
--- a/src/kpagewidgetmodel.cpp
+++ b/src/kpagewidgetmodel.cpp
@@ -332,7 +332,7 @@
 Qt::ItemFlags KPageWidgetModel::flags(const QModelIndex &index) const
 {
     if (!index.isValid()) {
-        return nullptr;
+        return Qt::NoItemFlags;
     }
 
     Qt::ItemFlags flags = Qt::ItemIsSelectable;
diff --git a/src/kpassworddialog.h b/src/kpassworddialog.h
--- a/src/kpassworddialog.h
+++ b/src/kpassworddialog.h
@@ -130,7 +130,7 @@
      * @param flags a set of KPasswordDialogFlag flags
      */
     explicit KPasswordDialog(QWidget *parent = nullptr,
-                             const KPasswordDialogFlags &flags = nullptr);
+                             const KPasswordDialogFlags &flags = KPasswordDialog::NoFlags);
 
     /**
      * Destructor
diff --git a/src/kruler.h b/src/kruler.h
--- a/src/kruler.h
+++ b/src/kruler.h
@@ -104,7 +104,7 @@
      * @param f          Will be handed over to QFrame.
      *
      **/
-    explicit KRuler(Qt::Orientation orient, QWidget *parent = nullptr, Qt::WindowFlags f = nullptr);
+    explicit KRuler(Qt::Orientation orient, QWidget *parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags());
 
     /**
      * Constructs a ruler with orientation @p orient and initial width @p widgetWidth.
@@ -121,7 +121,7 @@
      *
      */
     KRuler(Qt::Orientation orient, int widgetWidth, QWidget *parent = nullptr,
-           Qt::WindowFlags f = nullptr);
+           Qt::WindowFlags f = Qt::WindowFlags());
 
     /**
      * Destructor.
diff --git a/src/kseparator.h b/src/kseparator.h
--- a/src/kseparator.h
+++ b/src/kseparator.h
@@ -44,16 +44,16 @@
      * @param parent parent object.
      * @param f extra QWidget flags.
      **/
-    explicit KSeparator(QWidget *parent = nullptr, Qt::WindowFlags f = nullptr);
+    explicit KSeparator(QWidget *parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags());
 
     /**
      * Constructor.
      * @param orientation Set the orientation of the separator.
      * Possible values are Horizontal or Vertical.
      * @param parent parent object.
      * @param f extra QWidget flags.
      **/
-    explicit KSeparator(Qt::Orientation orientation, QWidget *parent = nullptr, Qt::WindowFlags f = nullptr);
+    explicit KSeparator(Qt::Orientation orientation, QWidget *parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags());
 
     /**
      * Returns the orientation of the separator.