diff --git a/src/corelib/serialization/qcbormap.cpp b/src/corelib/serialization/qcbormap.cpp
index 56068e333dc..3934c964e40 100644
--- a/src/corelib/serialization/qcbormap.cpp
+++ b/src/corelib/serialization/qcbormap.cpp
@@ -422,14 +422,7 @@ QList<QCborValue> QCborMap::keys() const
  */
 QCborValueRef QCborMap::operator[](qint64 key)
 {
-    auto it = find(key);
-    if (it == constEnd()) {
-        // insert element
-        detach(it.item.i + 2);
-        d->append(key);
-        d->append(Undefined{});
-    }
-    return { d.data(), it.item.i };
+    return QCborContainerPrivate::findOrAddMapKey(*this, key);
 }
 
 /*!
@@ -549,14 +542,7 @@ QCborValueRef QCborMap::operator[](qint64 key)
  */
 QCborValueRef QCborMap::operator[](QLatin1String key)
 {
-    auto it = find(key);
-    if (it == constEnd()) {
-        // insert element
-        detach(it.item.i + 2);
-        d->append(key);
-        d->append(Undefined{});
-    }
-    return { d.data(), it.item.i };
+    return QCborContainerPrivate::findOrAddMapKey(*this, key);
 }
 
 /*!
@@ -678,14 +664,7 @@ QCborValueRef QCborMap::operator[](QLatin1String key)
  */
 QCborValueRef QCborMap::operator[](const QString & key)
 {
-    auto it = find(key);
-    if (it == constEnd()) {
-        // insert element
-        detach(it.item.i + 2);
-        d->append(key);
-        d->append(Undefined{});
-    }
-    return { d.data(), it.item.i };
+    return QCborContainerPrivate::findOrAddMapKey(*this, qToStringViewIgnoringNull(key));
 }
 
 /*!
@@ -803,14 +782,15 @@ QCborValueRef QCborMap::operator[](const QString & key)
  */
 QCborValueRef QCborMap::operator[](const QCborValue &key)
 {
-    auto it = find(key);
-    if (it == constEnd()) {
-        // insert element
-        detach(it.item.i + 2);
-        d->append(key);
-        d->append(Undefined{});
-    }
-    return { d.data(), it.item.i };
+    return QCborContainerPrivate::findOrAddMapKey<const QCborValue &>(*this, key);
+}
+
+template <typename KeyType> inline QCborValueRef
+QCborContainerPrivate::findOrAddMapKey(QCborMap &map, KeyType key)
+{
+    QCborValueRef result = findOrAddMapKey<KeyType>(map.d.data(), key);
+    map.d = result.d;
+    return result;
 }
 
 /*!
diff --git a/src/corelib/serialization/qcborvalue_p.h b/src/corelib/serialization/qcborvalue_p.h
index 55a5dbf8717..185b74bc932 100644
--- a/src/corelib/serialization/qcborvalue_p.h
+++ b/src/corelib/serialization/qcborvalue_p.h
@@ -439,6 +439,30 @@ public:
         }
         return QCborValueRef{ this, i + 1 };
     }
+    template <typename KeyType> static QCborValueRef
+    findOrAddMapKey(QCborContainerPrivate *container, KeyType key)
+    {
+        qsizetype size = 0;
+        qsizetype index = size + 1;
+        if (container) {
+            size = container->elements.size();
+            index = container->findCborMapKey<KeyType>(key).i; // returns size + 1 if not found
+        }
+        Q_ASSERT(index & 1);
+        Q_ASSERT((size & 1) == 0);
+
+        container = detach(container, qMax(index + 1, size));
+        Q_ASSERT(container);
+        Q_ASSERT((container->elements.size() & 1) == 0);
+
+        if (index >= size) {
+            container->append(key);
+            container->append(QCborValue());
+        }
+        Q_ASSERT(index < container->elements.size());
+        return { container, index };
+    }
+    template <typename KeyType> static QCborValueRef findOrAddMapKey(QCborMap &map, KeyType key);
 
 #if QT_CONFIG(cborstreamreader)
     void decodeValueFromCbor(QCborStreamReader &reader, int remainingStackDepth);