diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 9cecd7b8269..d06ac392a89 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -2882,6 +2882,10 @@ QString &QString::append(const QString &str)
     if (!str.isNull()) {
         if (isNull()) {
             operator=(str);
+            if (Q_UNLIKELY(!d.isMutable() && d.size > 0)) {
+                d.detach(); // fromRawData, so we do a deep copy
+                d.data()[d.size] = u'\0';
+            }
         } else if (str.size()) {
             append(str.constData(), str.size());
         }
diff --git a/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp b/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
index cd750aa268b..f38c8ee681e 100644
--- a/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
+++ b/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
@@ -81,6 +81,7 @@ private slots:
     void prependExtended_data();
     void prependExtended();
     void append();
+    void appendFromRawData();
     void appendExtended_data();
     void appendExtended();
     void insert();
@@ -1142,6 +1143,20 @@ void tst_QByteArray::append()
     }
 }
 
+void tst_QByteArray::appendFromRawData()
+{
+    char rawData[] = "Hello World!";
+    QByteArray ba = QByteArray::fromRawData(rawData, std::size(rawData) - 1);
+
+    QByteArray copy;
+    copy.append(ba);
+    QCOMPARE(copy, ba);
+    // We make an _actual_ copy, because appending a byte array
+    // created with fromRawData() might be optimized to copy the DataPointer,
+    // which means we may point to temporary stack data.
+    QVERIFY(copy.constData() != ba.constData());
+}
+
 void tst_QByteArray::appendExtended_data()
 {
     prependExtended_data();
diff --git a/tests/auto/corelib/text/qstring/tst_qstring.cpp b/tests/auto/corelib/text/qstring/tst_qstring.cpp
index 861f2b77e21..7ea7c4ba0aa 100644
--- a/tests/auto/corelib/text/qstring/tst_qstring.cpp
+++ b/tests/auto/corelib/text/qstring/tst_qstring.cpp
@@ -434,6 +434,8 @@ private slots:
     void append_bytearray_special_cases_data();
     void append_bytearray_special_cases();
 
+    void appendFromRawData();
+
     void operator_pluseq_qstring()            { operator_pluseq_impl<QString>(); }
     void operator_pluseq_qstring_data()       { operator_pluseq_data(); }
     void operator_pluseq_qstringview()        { operator_pluseq_impl<QStringView, QString &(QString::*)(QStringView)>(); }
@@ -3015,6 +3017,21 @@ void tst_QString::append_bytearray_special_cases()
     }
 }
 
+void tst_QString::appendFromRawData()
+{
+    const char16_t utf[] = u"Hello World!";
+    auto *rawData = reinterpret_cast<const QChar *>(utf);
+    QString str = QString::fromRawData(rawData, std::size(utf) - 1);
+
+    QString copy;
+    copy.append(str);
+    QCOMPARE(copy, str);
+    // We make an _actual_ copy, because appending a byte array
+    // created with fromRawData() might be optimized to copy the DataPointer,
+    // which means we may point to temporary stack data.
+    QVERIFY(copy.constData() != str.constData());
+}
+
 void tst_QString::operator_pluseq_special_cases()
 {
     {