diff --git a/mitmproxy/tools/web/app.py b/mitmproxy/tools/web/app.py
index 761c23830..7ea24351a 100644
--- a/mitmproxy/tools/web/app.py
+++ b/mitmproxy/tools/web/app.py
@@ -308,7 +308,7 @@ class WebSocketEventBroadcaster(tornado.websocket.WebSocketHandler):
             "utf8", "surrogateescape"
         )
 
-        for conn in cls.connections:
+        for conn in cls.connections.copy():
             cls.send(conn, message)