fix changed set bug

1 files changed, 1 insertions, 1 deletions

diff --git a/at/web.py b/at/web.py
index 4a3d298..e758390 100644
--- a/at/web.py
+++ b/at/web.py
@@ -131,7 +131,7 @@ def app(instance_path, devices_api, config):
         for name, prefixes in app.config['SPECIAL_DEVICES'].items():
             result[name] = set()
             prefixes = tuple(prefixes) # startswith accepts tuple as argument
-            for hwaddr in unknown:
+            for hwaddr in list(unknown):
                 if hwaddr.startswith(prefixes):
                     result[name].add(hwaddr)
                     unknown.discard(hwaddr)