diff --git a/server/heartbeat_diagnostics_stub.py b/server/heartbeat_diagnostics_stub.py
index 30e7943..6c78932 100644
--- a/server/heartbeat_diagnostics_stub.py
+++ b/server/heartbeat_diagnostics_stub.py
@@ -70,13 +70,15 @@ def store_heartbeat_diagnostics(
         else None
     )
     cursor = db.cursor()
+    # COALESCE preserves existing column values when the v1.0.0 payload omits
+    # diagnostic fields (Pydantic resolves them to None).
     cursor.execute(
         """UPDATE heartbeats
-              SET reset_reason         = ?,
-                  heap_free            = ?,
-                  heap_min_free        = ?,
-                  last_disconnect_code = ?,
-                  recent_events        = ?
+              SET reset_reason         = COALESCE(?, reset_reason),
+                  heap_free            = COALESCE(?, heap_free),
+                  heap_min_free        = COALESCE(?, heap_min_free),
+                  last_disconnect_code = COALESCE(?, last_disconnect_code),
+                  recent_events        = COALESCE(?, recent_events)
             WHERE device_id = ?""",
         (
             hb.reset_reason,