diff --git a/src/gui/painting/qtransform.cpp b/src/gui/painting/qtransform.cpp
index c0e2bf6e2fe..768e92a15bf 100644
--- a/src/gui/painting/qtransform.cpp
+++ b/src/gui/painting/qtransform.cpp
@@ -1609,7 +1609,7 @@ QPolygon QTransform::mapToPolygon(const QRect &rect) const
 */
 bool QTransform::squareToQuad(const QPolygonF &quad, QTransform &trans)
 {
-    if (quad.size() != 4)
+    if (quad.size() != (quad.isClosed() ? 5 : 4))
         return false;
 
     qreal dx0 = quad[0].x();
diff --git a/tests/auto/gui/painting/qtransform/tst_qtransform.cpp b/tests/auto/gui/painting/qtransform/tst_qtransform.cpp
index 9da3cb568dd..73b66775cea 100644
--- a/tests/auto/gui/painting/qtransform/tst_qtransform.cpp
+++ b/tests/auto/gui/painting/qtransform/tst_qtransform.cpp
@@ -35,6 +35,7 @@ private slots:
     void mapInt();
     void mapPathWithPoint();
     void mapRectToPolygon(); // QTBUG-127723
+    void quadToQuad(); // QTBUG-21329
 
 private:
     void mapping_data();
@@ -714,6 +715,14 @@ void tst_QTransform::mapRectToPolygon()
     QCOMPARE(polygon1, polygon2);
 }
 
+void tst_QTransform::quadToQuad() // QTBUG-21329
+{
+    QTransform result;
+    QVERIFY(QTransform::quadToQuad(QRectF(0, 0, 1, 1), QRectF(0, 0, 1, 1), result));
+    QPolygonF trapezoid({{0, 0}, {10, 0}, {11, 11}, {0, 10}});
+    QVERIFY(QTransform::quadToQuad(trapezoid, trapezoid.boundingRect(), result));
+}
+
 QTEST_APPLESS_MAIN(tst_QTransform)