diff --git a/dronekit/__init__.py b/dronekit/__init__.py
index 4f3613906..65ec9af10 100644
--- a/dronekit/__init__.py
+++ b/dronekit/__init__.py
@@ -789,10 +789,10 @@ def __len__(self):
     def _update_channel(self, channel, value):
         # If we have channels on different ports, we expand the Channels
         # object to support them.
-        channel = int(channel)
         self._readonly = False
         self[channel] = value
         self._readonly = True
+        channel = int(channel)
         self._count = max(self._count, channel)
 
     @property