diff --git a/src/TgSharp.Core/Network/MtProtoSender.cs b/src/TgSharp.Core/Network/MtProtoSender.cs
index b1699ce..04295a2 100644
--- a/src/TgSharp.Core/Network/MtProtoSender.cs
+++ b/src/TgSharp.Core/Network/MtProtoSender.cs
@@ -37,7 +37,9 @@ namespace TgSharp.Core.Network
 
         private int GenerateSequence(bool confirmed)
         {
-            return confirmed ? session.Sequence++ * 2 + 1 : session.Sequence * 2;
+            lock (session.Lock) {
+                return confirmed ? session.Sequence++ * 2 + 1 : session.Sequence * 2;
+            }
         }
 
         public async Task Send(TLMethod request, CancellationToken token = default(CancellationToken))
diff --git a/src/TgSharp.Core/Session.cs b/src/TgSharp.Core/Session.cs
index 93db017..c20cdd5 100644
--- a/src/TgSharp.Core/Session.cs
+++ b/src/TgSharp.Core/Session.cs
@@ -52,6 +52,8 @@ namespace TgSharp.Core
 
     public class Session
     {
+        internal object Lock = new object ();
+
         public int Sequence { get; set; }
 #if CI
             // see the same CI-wrapped assignment in .FromBytes(), but this one will become useful