diff --git a/docs/ChatGptNet/IChatGptClient/AddFunctionResponseAsync.md b/docs/ChatGptNet/IChatGptClient/AddFunctionResponseAsync.md
index 5abdc79..52268c5 100644
--- a/docs/ChatGptNet/IChatGptClient/AddFunctionResponseAsync.md
+++ b/docs/ChatGptNet/IChatGptClient/AddFunctionResponseAsync.md
@@ -22,6 +22,7 @@ The Task corresponding to the asynchronous operation.
 
 | exception | condition |
 | --- | --- |
+| ArgumentNullException | *functionName* or *content* are `null`. |
 | InvalidOperationException | The conversation history is empty. |
 
 ## See Also
diff --git a/src/ChatGptNet/ChatGptClient.cs b/src/ChatGptNet/ChatGptClient.cs
index d7d4e76..d7e84e2 100644
--- a/src/ChatGptNet/ChatGptClient.cs
+++ b/src/ChatGptNet/ChatGptClient.cs
@@ -221,22 +221,17 @@ public async Task<Guid> LoadConversationAsync(Guid conversationId, IEnumerable<C
         // Ensures that conversationId isn't empty.
         conversationId = (conversationId == Guid.Empty) ? Guid.NewGuid() : conversationId;
 
-        if (replaceHistory)
+        // If messages must replace history, just use the current list, discarding all the previously cached content.
+        if (!replaceHistory)
         {
-            // If messages must replace history, just use the current list, discarding all the previously cached content.
-            // If messages.Count() > ChatGptOptions.MessageLimit, the UpdateCacheAsync method takes care of taking only the last messages.
-            await UpdateCacheAsync(conversationId, messages, cancellationToken);
-        }
-        else
-        {
-            // Retrieves the current history and adds new messages.
+            // Otherwise, retrieves the current history and adds the messages.
             var conversationHistory = await cache.GetAsync(conversationId, cancellationToken) ?? Enumerable.Empty<ChatGptMessage>();
-            conversationHistory = conversationHistory.Union(messages);
-
-            // If messages.Count() > ChatGptOptions.MessageLimit, the UpdateCacheAsync method takes care of taking only the last messages.
-            await UpdateCacheAsync(conversationId, conversationHistory, cancellationToken);
+            messages = conversationHistory.Union(messages);
         }
 
+        // If messages.Count() > ChatGptOptions.MessageLimit, the UpdateCacheAsync method takes care of taking only the last messages.
+        await UpdateCacheAsync(conversationId, messages.ToList(), cancellationToken);
+
         return conversationId;
     }
 
@@ -265,6 +260,9 @@ public async Task AddInteractionAsync(Guid conversationId, string question, stri
 
     public async Task AddFunctionResponseAsync(Guid conversationId, string functionName, string content, CancellationToken cancellationToken = default)
     {
+        ArgumentNullException.ThrowIfNull(functionName);
+        ArgumentNullException.ThrowIfNull(content);
+
         var messages = await cache.GetAsync(conversationId, cancellationToken);
         if (!messages?.Any() ?? true)
         {
diff --git a/src/ChatGptNet/IChatGptClient.cs b/src/ChatGptNet/IChatGptClient.cs
index c3de536..d47d82a 100644
--- a/src/ChatGptNet/IChatGptClient.cs
+++ b/src/ChatGptNet/IChatGptClient.cs
@@ -226,6 +226,7 @@ Task<Guid> LoadConversationAsync(IEnumerable<ChatGptMessage> messages, Cancellat
     /// <param name="content">The content of the function response.</param>
     /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
     /// <returns>The <see cref="Task"/> corresponding to the asynchronous operation.</returns>
+    /// <exception cref="ArgumentNullException"><paramref name="functionName"/> or <paramref name="content"/> are <see langword="null"/>.</exception>
     /// <exception cref="InvalidOperationException">The conversation history is empty.</exception>
     /// <seealso  cref="AskAsync(Guid, string, ChatGptParameters?, string?, bool, CancellationToken)"/>
     /// <seealso cref="AskStreamAsync(Guid, string, ChatGptParameters?, string?, bool, CancellationToken)"/>