diff --git a/src/corelib/tools/qline.cpp b/src/corelib/tools/qline.cpp
index da66ada3731..e652f84772d 100644
--- a/src/corelib/tools/qline.cpp
+++ b/src/corelib/tools/qline.cpp
@@ -40,7 +40,6 @@
 #include "qline.h"
 #include "qdebug.h"
 #include "qdatastream.h"
-#include "qmath.h"
 #include <private/qnumeric_p.h>
 
 QT_BEGIN_NAMESPACE
diff --git a/src/corelib/tools/qline.h b/src/corelib/tools/qline.h
index 066bcecc1b2..8889c4afb9b 100644
--- a/src/corelib/tools/qline.h
+++ b/src/corelib/tools/qline.h
@@ -40,6 +40,8 @@
 #ifndef QLINE_H
 #define QLINE_H
 
+#include "qmath.h"
+
 #include <QtCore/qpoint.h>
 
 QT_BEGIN_NAMESPACE
@@ -369,12 +371,15 @@ constexpr inline QPointF QLineF::center() const
     return QPointF(0.5 * pt1.x() + 0.5 * pt2.x(), 0.5 * pt1.y() + 0.5 * pt2.y());
 }
 
+QT_WARNING_DISABLE_FLOAT_COMPARE
+
 inline void QLineF::setLength(qreal len)
 {
-    if (isNull())
+    const qreal oldlength = qSqrt(dx() * dx() + dy() * dy());
+    if (!oldlength)
         return;
-    QLineF v = unitVector();
-    pt2 = QPointF(pt1.x() + v.dx() * len, pt1.y() + v.dy() * len);
+    const qreal factor = len / oldlength;
+    pt2 = QPointF(pt1.x() + dx() * factor, pt1.y() + dy() * factor);
 }
 
 constexpr inline QPointF QLineF::pointAt(qreal t) const
diff --git a/tests/auto/corelib/tools/qline/tst_qline.cpp b/tests/auto/corelib/tools/qline/tst_qline.cpp
index 4f3be0f3d99..7e07e9d1e21 100644
--- a/tests/auto/corelib/tools/qline/tst_qline.cpp
+++ b/tests/auto/corelib/tools/qline/tst_qline.cpp
@@ -40,6 +40,8 @@ private slots:
     void testLength();
     void testLength_data();
 
+    void testSetLength();
+
     void testCenter();
     void testCenter_data();
 
@@ -258,6 +260,15 @@ void tst_QLine::testLength()
     QCOMPARE(l.dy(), qreal(vy));
 }
 
+void tst_QLine::testSetLength()
+{
+    QLineF l(0, 0, 4e-323, 5e-324);
+    const qreal newLength = 1892;
+    const qreal oldLength = l.length();
+    l.setLength(newLength);
+    QCOMPARE(l.length(), oldLength ? newLength : 0);
+}
+
 void tst_QLine::testCenter()
 {
     QFETCH(int, x1);