diff --git a/src/ArduinoJson/Memory/MemoryPoolImpl.hpp b/src/ArduinoJson/Memory/MemoryPoolImpl.hpp
index 2046aa55..a8dbdf4c 100644
--- a/src/ArduinoJson/Memory/MemoryPoolImpl.hpp
+++ b/src/ArduinoJson/Memory/MemoryPoolImpl.hpp
@@ -69,12 +69,12 @@ inline SlotWithId MemoryPoolList::allocFromFreeList() {
   ARDUINOJSON_ASSERT(freeList_ != NULL_SLOT);
   auto id = freeList_;
   auto slot = getSlot(freeList_);
-  freeList_ = slot->free.next;
+  freeList_ = reinterpret_cast<FreeSlot*>(slot)->next;
   return {slot, id};
 }
 
 inline void MemoryPoolList::freeSlot(SlotWithId slot) {
-  slot->free.next = freeList_;
+  reinterpret_cast<FreeSlot*>(slot.slot())->next = freeList_;
   freeList_ = slot.id();
 }
 
diff --git a/src/ArduinoJson/Memory/MemoryPoolList.hpp b/src/ArduinoJson/Memory/MemoryPoolList.hpp
index b87b047f..b2eae0da 100644
--- a/src/ArduinoJson/Memory/MemoryPoolList.hpp
+++ b/src/ArduinoJson/Memory/MemoryPoolList.hpp
@@ -15,6 +15,10 @@ ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 using PoolCount = SlotId;
 
 class MemoryPoolList {
+  struct FreeSlot {
+    SlotId next;
+  };
+
  public:
   MemoryPoolList() = default;
 
diff --git a/src/ArduinoJson/Variant/VariantSlot.hpp b/src/ArduinoJson/Variant/VariantSlot.hpp
index cdbd8863..efde6583 100644
--- a/src/ArduinoJson/Variant/VariantSlot.hpp
+++ b/src/ArduinoJson/Variant/VariantSlot.hpp
@@ -13,15 +13,10 @@ ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
 struct StringNode;
 
-struct FreeSlot {
-  SlotId next;
-};
-
 union VariantSlot {
   VariantSlot() {}
 
   VariantData variant;
-  FreeSlot free;
 };
 
 ARDUINOJSON_END_PRIVATE_NAMESPACE