diff --git a/src/ArduinoJson/Json/JsonSerializer.hpp b/src/ArduinoJson/Json/JsonSerializer.hpp
index 7c6f6424..200c7f3a 100644
--- a/src/ArduinoJson/Json/JsonSerializer.hpp
+++ b/src/ArduinoJson/Json/JsonSerializer.hpp
@@ -22,7 +22,7 @@ class JsonSerializer : public Visitor<size_t> {
   FORCE_INLINE size_t visitArray(const CollectionData &array) {
     write('[');
 
-    VariantSlot *slot = array.head();
+    const VariantSlot *slot = array.head();
 
     while (slot != 0) {
       slot->data()->resolve()->accept(*this);
@@ -41,7 +41,7 @@ class JsonSerializer : public Visitor<size_t> {
   size_t visitObject(const CollectionData &object) {
     write('{');
 
-    VariantSlot *slot = object.head();
+    const VariantSlot *slot = object.head();
 
     while (slot != 0) {
       _formatter.writeString(slot->key());
diff --git a/src/ArduinoJson/Json/PrettyJsonSerializer.hpp b/src/ArduinoJson/Json/PrettyJsonSerializer.hpp
index 3f7fd262..9485cd75 100644
--- a/src/ArduinoJson/Json/PrettyJsonSerializer.hpp
+++ b/src/ArduinoJson/Json/PrettyJsonSerializer.hpp
@@ -19,7 +19,7 @@ class PrettyJsonSerializer : public JsonSerializer<TWriter> {
   PrettyJsonSerializer(TWriter writer) : base(writer), _nesting(0) {}
 
   size_t visitArray(const CollectionData &array) {
-    VariantSlot *slot = array.head();
+    const VariantSlot *slot = array.head();
     if (slot) {
       base::write("[\r\n");
       _nesting++;
@@ -40,7 +40,7 @@ class PrettyJsonSerializer : public JsonSerializer<TWriter> {
   }
 
   size_t visitObject(const CollectionData &object) {
-    VariantSlot *slot = object.head();
+    const VariantSlot *slot = object.head();
     if (slot) {
       base::write("{\r\n");
       _nesting++;
diff --git a/src/ArduinoJson/MsgPack/MsgPackSerializer.hpp b/src/ArduinoJson/MsgPack/MsgPackSerializer.hpp
index 8cf99801..d70e9c98 100644
--- a/src/ArduinoJson/MsgPack/MsgPackSerializer.hpp
+++ b/src/ArduinoJson/MsgPack/MsgPackSerializer.hpp
@@ -55,7 +55,7 @@ class MsgPackSerializer : public Visitor<size_t> {
       writeByte(0xDD);
       writeInteger(uint32_t(n));
     }
-    for (VariantSlot* slot = array.head(); slot; slot = slot->next()) {
+    for (const VariantSlot* slot = array.head(); slot; slot = slot->next()) {
       slot->data()->resolve()->accept(*this);
     }
     return bytesWritten();
@@ -72,7 +72,7 @@ class MsgPackSerializer : public Visitor<size_t> {
       writeByte(0xDF);
       writeInteger(uint32_t(n));
     }
-    for (VariantSlot* slot = object.head(); slot; slot = slot->next()) {
+    for (const VariantSlot* slot = object.head(); slot; slot = slot->next()) {
       visitString(slot->key());
       slot->data()->resolve()->accept(*this);
     }