diff --git a/src/widgets/widgets/qmdisubwindow.cpp b/src/widgets/widgets/qmdisubwindow.cpp
index 2340c8fe4b..41e32028d9 100644
--- a/src/widgets/widgets/qmdisubwindow.cpp
+++ b/src/widgets/widgets/qmdisubwindow.cpp
@@ -3058,6 +3058,9 @@ void QMdiSubWindow::leaveEvent(QEvent * /*leaveEvent*/)
 
 /*!
     \reimp
+
+    \warning When maximizing or restoring a subwindow, the resulting call to this function
+    may have an invalid QResizeEvent::oldSize().
 */
 void QMdiSubWindow::resizeEvent(QResizeEvent *resizeEvent)
 {