diff --git a/at.py b/at.py
index 8248bd7..9e1dfa0 100644
--- a/at.py
+++ b/at.py
@@ -262,15 +262,15 @@ def now_at():
     devices = app.updater.get_active_devices()
     device_infos = list(get_device_infos(g.db, devices.keys()))
     device_infos.sort(key=lambda di: devices.__getitem__)
-    all_unknown = set(devices.keys()) - set(d.hwaddr for d in device_infos)
+    unknown = set(devices.keys()) - set(d.hwaddr for d in device_infos)
     # das kektop sorting maschine
-    unknown = all_unknown
     for name, prefixes in app.config['SPECIAL_DEVICES'].items():
         result[name] = set()
-        for u in all_unknown:
+        for u in unknown.copy():
             if u.startswith(prefixes):
                 result[name].add(u)
                 unknown.discard(u)
+
     result['unknown'] = unknown
 
     users = {}