diff --git a/src/Cli/dotnet/Commands/Test/MTP/IPC/Serializers/UnknownMessageSerializer.cs b/src/Cli/dotnet/Commands/Test/MTP/IPC/Serializers/UnknownMessageSerializer.cs
index 3ebc6f1802be..f1e3331f333b 100644
--- a/src/Cli/dotnet/Commands/Test/MTP/IPC/Serializers/UnknownMessageSerializer.cs
+++ b/src/Cli/dotnet/Commands/Test/MTP/IPC/Serializers/UnknownMessageSerializer.cs
@@ -17,7 +17,5 @@ public object Deserialize(Stream _)
     }
 
     public void Serialize(object _, Stream stream)
-    {
-        WriteInt(stream, Id);
-    }
+        => Environment.FailFast("SDK never serializes UnknownMessage. This should never be hit.");
 }