diff --git a/thorlcr/shared/thwatchdog.hpp b/thorlcr/shared/thwatchdog.hpp
index 2e65594d06d..ba8b5edf0be 100644
--- a/thorlcr/shared/thwatchdog.hpp
+++ b/thorlcr/shared/thwatchdog.hpp
@@ -30,11 +30,13 @@
 struct HeartBeatPacketHeader
 {
 public:
+    // NB: packetSize and progressSize are back-patched and must remain at fixed offsets from the start of this struct
     size32_t packetSize = 0;   // used as validity check must be first
-    SocketEndpoint sender;
-    unsigned tick = 0;         // sequence check
     size32_t progressSize = 0; // size of progress data (following performance data)
 
+    unsigned tick = 0;         // sequence check
+    SocketEndpoint sender;
+
 public:
     void serialize(MemoryBuffer & out) const
     {