diff --git a/toonz/sources/include/toonzqt/glwidget_for_highdpi.h b/toonz/sources/include/toonzqt/glwidget_for_highdpi.h
index 2bee6b4..258e17c 100644
--- a/toonz/sources/include/toonzqt/glwidget_for_highdpi.h
+++ b/toonz/sources/include/toonzqt/glwidget_for_highdpi.h
@@ -11,7 +11,7 @@
 
 class GLWidgetForHighDpi : public QOpenGLWidget, protected QOpenGLFunctions {
 public:
-  GLWidgetForHighDpi(QWidget *parent   = Q_NULLPTR,
+  GLWidgetForHighDpi(QWidget *parent   = nullptr,
                      Qt::WindowFlags f = Qt::WindowFlags())
       : QOpenGLWidget(parent, f) {}
 
diff --git a/toonz/sources/include/toonzqt/stylenameeditor.h b/toonz/sources/include/toonzqt/stylenameeditor.h
index 2852bf8..1865e5c 100644
--- a/toonz/sources/include/toonzqt/stylenameeditor.h
+++ b/toonz/sources/include/toonzqt/stylenameeditor.h
@@ -24,7 +24,7 @@ class NewWordDialog : public QDialog {
   QLineEdit *m_lineEdit;
 
 public:
-  NewWordDialog(QWidget *parent = Q_NULLPTR);
+  NewWordDialog(QWidget *parent = nullptr);
   QString getName();
 };
 
@@ -33,7 +33,7 @@ public:
 class WordButton : public QPushButton {
   Q_OBJECT
 public:
-  WordButton(const QString &text, QWidget *parent = Q_NULLPTR);
+  WordButton(const QString &text, QWidget *parent = nullptr);
 
 protected:
   void contextMenuEvent(QContextMenuEvent *) override;
@@ -54,7 +54,7 @@ class AddWordButton final : public WordButton {
   int m_column;
 
 public:
-  AddWordButton(const int col, QWidget *parent = Q_NULLPTR);
+  AddWordButton(const int col, QWidget *parent = nullptr);
 
 protected slots:
   void onClicked() override;
diff --git a/toonz/sources/tnztools/tooloptionscontrols.h b/toonz/sources/tnztools/tooloptionscontrols.h
index 6e733c6..6af4bbc 100644
--- a/toonz/sources/tnztools/tooloptionscontrols.h
+++ b/toonz/sources/tnztools/tooloptionscontrols.h
@@ -603,7 +603,7 @@ protected:
   void mouseReleaseEvent(QMouseEvent *) override;
 
 public:
-  ClickableLabel(const QString &text, QWidget *parent = Q_NULLPTR,
+  ClickableLabel(const QString &text, QWidget *parent = nullptr,
                  Qt::WindowFlags f = Qt::WindowFlags());
   ~ClickableLabel();