diff --git a/src/corelib/text/qstringconverter.h b/src/corelib/text/qstringconverter.h
index 213de9fe55..c1645f0938 100644
--- a/src/corelib/text/qstringconverter.h
+++ b/src/corelib/text/qstringconverter.h
@@ -62,12 +62,23 @@ public:
 #endif
 
     qsizetype requiredSpace(qsizetype inputLength) const
-    { return iface->fromUtf16Len(inputLength); }
+    { return iface ? iface->fromUtf16Len(inputLength) : 0; }
     char *appendToBuffer(char *out, QStringView in)
-    { return iface->fromUtf16(out, in, &state); }
+    {
+        if (!iface) {
+            state.invalidChars = 1;
+            return out;
+        }
+        return iface->fromUtf16(out, in, &state);
+    }
 private:
     QByteArray encodeAsByteArray(QStringView in)
     {
+        if (!iface) {
+            // ensure that hasError returns true
+            state.invalidChars = 1;
+            return {};
+        }
         QByteArray result(iface->fromUtf16Len(in.size()), Qt::Uninitialized);
         char *out = result.data();
         out = iface->fromUtf16(out, in, &state);
@@ -120,12 +131,23 @@ public:
 #endif
 
     qsizetype requiredSpace(qsizetype inputLength) const
-    { return iface->toUtf16Len(inputLength); }
+    { return iface ? iface->toUtf16Len(inputLength) : 0; }
     QChar *appendToBuffer(QChar *out, QByteArrayView ba)
-    { return iface->toUtf16(out, ba, &state); }
+    {
+        if (!iface) {
+            state.invalidChars = 1;
+            return out;
+        }
+        return iface->toUtf16(out, ba, &state);
+    }
 private:
     QString decodeAsString(QByteArrayView in)
     {
+        if (!iface) {
+            // ensure that hasError returns true
+            state.invalidChars = 1;
+            return {};
+        }
         QString result(iface->toUtf16Len(in.size()), Qt::Uninitialized);
         const QChar *out = iface->toUtf16(result.data(), in, &state);
         result.truncate(out - result.constData());
diff --git a/tests/auto/corelib/text/qstringconverter/tst_qstringconverter.cpp b/tests/auto/corelib/text/qstringconverter/tst_qstringconverter.cpp
index 6fe20b1453..231df0390e 100644
--- a/tests/auto/corelib/text/qstringconverter/tst_qstringconverter.cpp
+++ b/tests/auto/corelib/text/qstringconverter/tst_qstringconverter.cpp
@@ -120,6 +120,8 @@ private slots:
 
     void constructByName();
 
+    void invalidConverter();
+
     void convertUtf8_data();
     void convertUtf8();
     void convertUtf8CharByChar_data() { convertUtf8_data(); }
@@ -181,6 +183,60 @@ void tst_QStringConverter::constructByName()
     QVERIFY(!strcmp(decoder.name(), "UTF-16"));
 }
 
+void tst_QStringConverter::invalidConverter()
+{
+    // QStringEncoder tests
+    {
+        QStringEncoder encoder;
+        QVERIFY(!encoder.isValid());
+        QVERIFY(!encoder.name());
+        QByteArray encoded = encoder(u"Some text");
+        QVERIFY(encoded.isEmpty());
+        QVERIFY(encoder.hasError());
+
+        encoder.resetState();
+        QVERIFY(!encoder.hasError());
+
+        encoded = encoder.encode(u"More text");
+        QVERIFY(encoded.isEmpty());
+        QVERIFY(encoder.hasError());
+        QCOMPARE(encoder.requiredSpace(42), 0);
+
+        encoder.resetState();
+        QVERIFY(!encoder.hasError());
+        char buffer[100];
+        char *position = encoder.appendToBuffer(buffer, u"Even more");
+        QCOMPARE(position, buffer);
+        QVERIFY(encoder.hasError());
+    }
+
+    // QStringDecoder tests
+    {
+        QStringDecoder decoder;
+        QVERIFY(!decoder.name());
+        QVERIFY(!decoder.isValid());
+        QString decoded = decoder("Some text");
+        QVERIFY(decoded.isEmpty());
+        QVERIFY(decoder.hasError());
+
+        decoder.resetState();
+        QVERIFY(!decoder.hasError());
+
+        decoded = decoder.decode("More text");
+        QVERIFY(decoded.isEmpty());
+        QVERIFY(decoder.hasError());
+
+        QCOMPARE(decoder.requiredSpace(42), 0);
+
+        decoder.resetState();
+        QVERIFY(!decoder.hasError());
+        QChar buffer[100];
+        QChar *position = decoder.appendToBuffer(buffer, "Even more");
+        QCOMPARE(position, buffer);
+        QVERIFY(decoder.hasError());
+    }
+}
+
 void tst_QStringConverter::convertUtf8_data()
 {
     QTest::addColumn<QStringConverter::Encoding>("encoding");