diff --git a/README.md b/README.md
index ef0c67c..847f343 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,8 @@
 
 # How to use
 
->⚠️ This library relies on asynchronous C# programming (`async/await`) so make sure you are familiar with this before proceeding.
+>⚠️ This library relies on asynchronous C# programming (`async/await`) so make sure you are familiar with this advanced topic before proceeding.  
+>If you are a beginner in C#, making a project based on this library might not be a great idea.
 
 After installing WTelegramClient through Nuget, your first Console program will be as simple as:
 ```csharp
diff --git a/src/Client.cs b/src/Client.cs
index 4b5ee27..550050c 100644
--- a/src/Client.cs
+++ b/src/Client.cs
@@ -1168,7 +1168,7 @@ namespace WTelegram
 		public Task<Message> SendMediaAsync(InputPeer peer, string caption, InputFileBase mediaFile, string mimeType = null, int reply_to_msg_id = 0, MessageEntity[] entities = null, DateTime schedule_date = default)
 		{
 			var filename = mediaFile is InputFile iFile ? iFile.name : (mediaFile as InputFileBig)?.name;
-			mimeType ??= Path.GetExtension(filename).ToLowerInvariant() switch
+			mimeType ??= Path.GetExtension(filename)?.ToLowerInvariant() switch
 			{
 				".jpg" or ".jpeg" or ".png" or ".bmp" => "photo",
 				".gif" => "image/gif",