diff --git a/Common/DStarDefines.h b/Common/DStarDefines.h
index cf5d356..6d1319e 100644
--- a/Common/DStarDefines.h
+++ b/Common/DStarDefines.h
@@ -1,5 +1,5 @@
 /*
- *	Copyright (C) 2009-2015,2018 by Jonathan Naylor, G4KLX
+ *	Copyright (C) 2009-2015,2018,2019 by Jonathan Naylor, G4KLX
  *
  *	This program is free software; you can redistribute it and/or modify
  *	it under the terms of the GNU General Public License as published by
@@ -111,6 +111,10 @@ const unsigned int  DSTAR_FRAMES_PER_SEC = 50U;
 const unsigned char SCRAMBLER_BYTE1 = 0x70U;
 const unsigned char SCRAMBLER_BYTE2 = 0x4FU;
 const unsigned char SCRAMBLER_BYTE3 = 0x93U;
+const unsigned char SCRAMBLER_BYTE4 = 0x40U;
+const unsigned char SCRAMBLER_BYTE5 = 0x64U;
+
+const unsigned char FAST_DATA_GUARD_BYTE = 0x02U;
 
 const unsigned int  DPLUS_PORT  = 20001U;
 const unsigned int  DEXTRA_PORT = 30001U;
diff --git a/Common/RepeaterHandler.cpp b/Common/RepeaterHandler.cpp
index 0736723..bcb958f 100644
--- a/Common/RepeaterHandler.cpp
+++ b/Common/RepeaterHandler.cpp
@@ -1,5 +1,5 @@
 /*
- *   Copyright (C) 2010-2015,2018 by Jonathan Naylor G4KLX
+ *   Copyright (C) 2010-2015,2018,2019 by Jonathan Naylor G4KLX
  *
  *   This program is free software; you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by
@@ -743,7 +743,8 @@ void CRepeaterHandler::processRepeater(CAMBEData& data)
 	// Check for the fast data signature
 	if (!m_fastData) {
 		unsigned char slowDataType = (buffer[VOICE_FRAME_LENGTH_BYTES] ^ SCRAMBLER_BYTE1) & SLOW_DATA_TYPE_MASK;
-		if (slowDataType == SLOW_DATA_TYPE_FAST_DATA1 || slowDataType == SLOW_DATA_TYPE_FAST_DATA2)
+		unsigned char guard = data[4U] ^ SCRAMBLER_BYTE5;
+		if ((slowDataType == SLOW_DATA_TYPE_FAST_DATA1 || slowDataType == SLOW_DATA_TYPE_FAST_DATA2) && guard == FAST_DATA_GUARD_BYTE) {
 			m_fastData = true;
 	}