diff --git a/include/ki/protocol/net/DMLSession.h b/include/ki/protocol/net/DMLSession.h
index 8245dc6..345b563 100644
--- a/include/ki/protocol/net/DMLSession.h
+++ b/include/ki/protocol/net/DMLSession.h
@@ -8,6 +8,12 @@ namespace protocol
 {
 namespace net
 {
+	enum class InvalidDMLMessageErrorCode
+	{
+		INVALID_MESSAGE_DATA,
+		INSUFFICIENT_ACCESS
+	};
+
 	/**
 	 * Implements an application protocol that uses the DML
 	 * message system (as seen in Wizard101 and Pirate101).
@@ -24,7 +30,7 @@ namespace net
 	protected:
 		void on_application_message(const PacketHeader& header) override;
 		virtual void on_message(const dml::Message &message) {}
-		virtual void on_invalid_message() {}
+		virtual void on_invalid_message(InvalidDMLMessageErrorCode error) {}
 	private:
 		const dml::MessageManager &m_manager;
 	};
diff --git a/src/protocol/net/DMLSession.cpp b/src/protocol/net/DMLSession.cpp
index f11a7ed..aec5c48 100644
--- a/src/protocol/net/DMLSession.cpp
+++ b/src/protocol/net/DMLSession.cpp
@@ -21,10 +21,11 @@ namespace net
 
 	void DMLSession::on_application_message(const PacketHeader& header)
 	{
+		// Attempt to create a Message instance from the data in the stream
 		const auto *message = m_manager.message_from_binary(m_data_stream);
 		if (!message)
 		{
-			on_invalid_message();
+			on_invalid_message(InvalidDMLMessageErrorCode::INVALID_MESSAGE_DATA);
 			return;
 		}
 
@@ -32,7 +33,7 @@ namespace net
 		if (get_access_level() >= message->get_access_level())
 			on_message(*message);
 		else
-			on_invalid_message();
+			on_invalid_message(InvalidDMLMessageErrorCode::INSUFFICIENT_ACCESS);
 		delete message;
 	}
 }