diff --git a/src/corelib/serialization/qjsonobject.cpp b/src/corelib/serialization/qjsonobject.cpp
index 99ab25f265..329bc4d2c9 100644
--- a/src/corelib/serialization/qjsonobject.cpp
+++ b/src/corelib/serialization/qjsonobject.cpp
@@ -1476,7 +1476,10 @@ void QJsonObject::setValueAt(int i, const QJsonValue &val)
     Q_ASSERT(o && i >= 0 && i < (int)o->length);
 
     QJsonPrivate::Entry *e = o->entryAt(i);
-    insertAt(i, e->key(), val, true);
+    if (val.t == QJsonValue::Undefined)
+        removeAt(i);
+    else
+        insertAt(i, e->key(), val, true);
 }
 
 /*!
diff --git a/tests/auto/corelib/serialization/json/tst_qtjson.cpp b/tests/auto/corelib/serialization/json/tst_qtjson.cpp
index 7b055b5b63..57aa67c142 100644
--- a/tests/auto/corelib/serialization/json/tst_qtjson.cpp
+++ b/tests/auto/corelib/serialization/json/tst_qtjson.cpp
@@ -1132,6 +1132,8 @@ void tst_QtJson::undefinedValues()
     QJsonObject object;
     object.insert("Key", QJsonValue(QJsonValue::Undefined));
     QCOMPARE(object.size(), 0);
+    object["Key"] = QJsonValue(QJsonValue::Undefined);
+    QCOMPARE(object.size(), 0);
 
     object.insert("Key", QLatin1String("Value"));
     QCOMPARE(object.size(), 1);