diff --git a/src/HarSharp/MessageBase.cs b/src/HarSharp/MessageBase.cs
index 07efd84..693ffc9 100644
--- a/src/HarSharp/MessageBase.cs
+++ b/src/HarSharp/MessageBase.cs
@@ -37,7 +37,7 @@ protected MessageBase()
         /// <summary>
         /// Gets or sets the total number of bytes from the start of the HTTP request message until (and including) the double CRLF before the body.
         /// </summary>
-        public int HeadersSize { get; set; }
+        public int? HeadersSize { get; set; }
 
         /// <summary>
         /// Gets or sets the size of the request body (POST data payload) in bytes.