diff --git a/dronekit/__init__.py b/dronekit/__init__.py
index dd7ab8252..db2b5039b 100644
--- a/dronekit/__init__.py
+++ b/dronekit/__init__.py
@@ -1161,7 +1161,9 @@ def listener(self, name, m):
             self._armed = (m.base_mode & mavutil.mavlink.MAV_MODE_FLAG_SAFETY_ARMED) != 0
             self.notify_attribute_listeners('armed', self.armed, cache=True)
             if self._master.mode_mapping() != None:
-                self._flightmode = {v: k for k, v in self._master.mode_mapping().items()}[m.custom_mode]
+                flightmodesById = {v: k for k, v in self._master.mode_mapping().items()}
+                if m.custom_mode in flightmodesById:
+                    self._flightmode = flightmodesById[m.custom_mode]
             self.notify_attribute_listeners('mode', self.mode, cache=True)
             self._system_status = m.system_status
             self.notify_attribute_listeners('system_status', self.system_status, cache=True)