diff --git a/control/walle.py b/control/walle.py
index fcac880..fd2c767 100644
--- a/control/walle.py
+++ b/control/walle.py
@@ -52,7 +52,8 @@ class WallE:
 
     def tick(self):
         remove = {}
-        for channel, target in self.servo_targets.items():
+        targets = list(self.servo_targets.items())
+        for channel, target in targets:
             current_value = self.servo_positions[channel]
             if target == current_value:
                 remove[channel] = target