diff --git a/src/abstractstringserialiser.cpp b/src/abstractstringserialiser.cpp
index e426eec..48adb25 100644
--- a/src/abstractstringserialiser.cpp
+++ b/src/abstractstringserialiser.cpp
@@ -47,32 +47,28 @@ QString AbstractStringSerialiserPrivate::guessDecimalsString(double val, QLocale
     return QString::number(val, 'f', guessDecimals(val));
 }
 
-QString AbstractStringSerialiserPrivate::variantToString(const QVariant &val)
+QString AbstractStringSerialiserPrivate::variantToString(const QVariant &val) const
 {
     QByteArray data;
     QDataStream outStream(&data, QIODevice::WriteOnly);
-#ifdef MS_DATASTREAM_VERSION
-    outStream.setVersion(MS_DATASTREAM_VERSION);
-#endif // MS_DATASTREAM_VERSION
+    outStream.setVersion(m_streamVersion);
     outStream << val;
     data = qCompress(data);
     return QString::fromLatin1(data.toBase64());
 }
 
-QVariant AbstractStringSerialiserPrivate::stringToVariant(const QString &val)
+QVariant AbstractStringSerialiserPrivate::stringToVariant(const QString &val) const
 {
     QByteArray data = QByteArray::fromBase64(val.toLatin1());
     data = qUncompress(data);
     QDataStream inStream(data);
-#ifdef MS_DATASTREAM_VERSION
-    inStream.setVersion(MS_DATASTREAM_VERSION);
-#endif // MS_DATASTREAM_VERSION
+    inStream.setVersion(m_streamVersion);
     QVariant result;
     inStream >> result;
     return result;
 }
 
-QVariant AbstractStringSerialiserPrivate::loadVariant(int type, const QString &val)
+QVariant AbstractStringSerialiserPrivate::loadVariant(int type, const QString &val) const
 {
     if (val.isEmpty())
         return QVariant();
@@ -150,7 +146,7 @@ QImage AbstractStringSerialiserPrivate::loadImageVariant(int type, const QString
 }
 #endif
 
-QString AbstractStringSerialiserPrivate::saveVariant(const QVariant &val)
+QString AbstractStringSerialiserPrivate::saveVariant(const QVariant &val) const
 {
     if (val.isNull())
         return QString();
diff --git a/src/htmlmodelserialiser.cpp b/src/htmlmodelserialiser.cpp
index 27ca57f..2f6c2f3 100644
--- a/src/htmlmodelserialiser.cpp
+++ b/src/htmlmodelserialiser.cpp
@@ -266,7 +266,7 @@ bool HtmlModelSerialiserPrivate::readHtml(QXmlStreamReader &reader)
     return true;
 }
 
-void HtmlModelSerialiserPrivate::writeHtmlVariant(QXmlStreamWriter &writer, const QVariant &val)
+void HtmlModelSerialiserPrivate::writeHtmlVariant(QXmlStreamWriter &writer, const QVariant &val) const
 {
     if (isImageType(val.userType())) {
         writer.writeEmptyElement(QStringLiteral("img"));
@@ -277,7 +277,7 @@ void HtmlModelSerialiserPrivate::writeHtmlVariant(QXmlStreamWriter &writer, cons
     writer.writeCharacters(saveVariant(val));
 }
 
-QVariant HtmlModelSerialiserPrivate::readHtmlVariant(QXmlStreamReader &reader, int valType)
+QVariant HtmlModelSerialiserPrivate::readHtmlVariant(QXmlStreamReader &reader, int valType) const
 {
     if (isImageType(valType)) {
         if (!reader.readNextStartElement())
diff --git a/src/private/abstractstringserialiser_p.h b/src/private/abstractstringserialiser_p.h
index 2cb8180..c7ab029 100644
--- a/src/private/abstractstringserialiser_p.h
+++ b/src/private/abstractstringserialiser_p.h
@@ -21,18 +21,18 @@ class QTextCodec;
 #endif
 class AbstractStringSerialiserPrivate : public AbstractModelSerialiserPrivate
 {
-    Q_DISABLE_COPY(AbstractStringSerialiserPrivate);
+    Q_DISABLE_COPY(AbstractStringSerialiserPrivate)
     Q_DECLARE_PUBLIC(AbstractStringSerialiser)
 protected:
     AbstractStringSerialiserPrivate(AbstractStringSerialiser *q);
-    static QString variantToString(const QVariant &val);
-    static QVariant stringToVariant(const QString &val);
+    QString variantToString(const QVariant &val) const;
+    QVariant stringToVariant(const QString &val) const;
 #ifdef QT_GUI_LIB
     static QImage loadImageVariant(int type, const QString &val);
     static QString saveImageVariant(const QImage &val);
 #endif
-    static QVariant loadVariant(int type, const QString &val);
-    static QString saveVariant(const QVariant &val);
+    QVariant loadVariant(int type, const QString &val) const;
+    QString saveVariant(const QVariant &val) const;
     static int guessDecimals(double val);
     static QString guessDecimalsString(double val, QLocale *loca = Q_NULLPTR);
 #if (QT_VERSION < QT_VERSION_CHECK(6, 0, 0))
diff --git a/src/private/htmlmodelserialiser_p.h b/src/private/htmlmodelserialiser_p.h
index 5a07301..a9ad89a 100644
--- a/src/private/htmlmodelserialiser_p.h
+++ b/src/private/htmlmodelserialiser_p.h
@@ -33,8 +33,8 @@ class HtmlModelSerialiserPrivate : public AbstractStringSerialiserPrivate
     {
         return val == QMetaType::QImage || val == QMetaType::QPixmap || val == QMetaType::QBitmap;
     }
-    static void writeHtmlVariant(QXmlStreamWriter &writer, const QVariant &val);
-    static QVariant readHtmlVariant(QXmlStreamReader &reader, int valType);
+    void writeHtmlVariant(QXmlStreamWriter &writer, const QVariant &val) const;
+    QVariant readHtmlVariant(QXmlStreamReader &reader, int valType) const;
     bool m_printStartDocument;
 };
 #endif // htmlmodelserialiser_p_h__