diff --git a/src/corelib/text/qstringconverter.cpp b/src/corelib/text/qstringconverter.cpp
index 1c51cf80c20..b17432c067a 100644
--- a/src/corelib/text/qstringconverter.cpp
+++ b/src/corelib/text/qstringconverter.cpp
@@ -2710,11 +2710,19 @@ QStringDecoder QStringDecoder::decoderForHtml(QByteArrayView data)
 #endif // !QT_BOOTSTRAPPED
 
 /*!
-    Returns the canonical name for encoding \a e.
+    Returns the canonical name for encoding \a e or \nullptr if \a e is an
+    invalid value.
+
+    \note In Qt versions prior to 6.10, 6.9.1, 6.8.4 or 6.5.9, calling this
+    function with an invalid argument resulted in undefined behavior. Since the
+    above-mentioned Qt versions, it returns nullptr instead.
 */
-const char *QStringConverter::nameForEncoding(QStringConverter::Encoding e)
+const char *QStringConverter::nameForEncoding(QStringConverter::Encoding e) noexcept
 {
-    return encodingInterfaces[int(e)].name;
+    auto i = size_t(e);
+    if (Q_UNLIKELY(i >= std::size(encodingInterfaces)))
+        return nullptr;
+    return encodingInterfaces[i].name;
 }
 
 /*!
diff --git a/src/corelib/text/qstringconverter_base.h b/src/corelib/text/qstringconverter_base.h
index 383eaa8a869..26ea9dfe0d7 100644
--- a/src/corelib/text/qstringconverter_base.h
+++ b/src/corelib/text/qstringconverter_base.h
@@ -162,7 +162,7 @@ public:
     Q_CORE_EXPORT static std::optional<Encoding> encodingForName(const char *name) noexcept;
 #endif
     Q_CORE_EXPORT static std::optional<Encoding> encodingForName(QAnyStringView name) noexcept;
-    Q_CORE_EXPORT static const char *nameForEncoding(Encoding e);
+    Q_DECL_PURE_FUNCTION Q_CORE_EXPORT static const char *nameForEncoding(Encoding e) noexcept;
     Q_CORE_EXPORT static std::optional<Encoding>
     encodingForData(QByteArrayView data, char16_t expectedFirstCharacter = 0) noexcept;
     Q_CORE_EXPORT static std::optional<Encoding> encodingForHtml(QByteArrayView data);