diff --git a/Chapter03/calendar_app.py b/Chapter03/calendar_app.py
index 1a1540a..274f2e8 100644
--- a/Chapter03/calendar_app.py
+++ b/Chapter03/calendar_app.py
@@ -33,6 +33,7 @@ def __init__(self):
             clicked=self.close
             )
         self.layout().addWidget(self.cancel_btn)
+        self.setWindowModality(qtc.Qt.ApplicationModal)
         self.show()
 
     @qtc.pyqtSlot()