diff --git a/libmproxy/protocol/http.py b/libmproxy/protocol/http.py
index b37ff7cf1..b345ee06d 100644
--- a/libmproxy/protocol/http.py
+++ b/libmproxy/protocol/http.py
@@ -239,7 +239,7 @@ class Http2Layer(_HttpLayer):
             return
         if isinstance(frame, PingFrame):
             # respond with pong
-            self.server_conn.send(PingFrame(flags=frame.Frame.FLAG_ACK, payload=frame.payload).to_bytes())
+            self.server_conn.send(PingFrame(flags=Frame.FLAG_ACK, payload=frame.payload).to_bytes())
             return
         self.log("Unexpected HTTP2 Frame: %s" % frame.human_readable(), "info")
 
@@ -251,7 +251,7 @@ class Http2Layer(_HttpLayer):
             return
         if isinstance(frame, PingFrame):
             # respond with pong
-            self.client_conn.send(PingFrame(flags=frame.Frame.FLAG_ACK, payload=frame.payload).to_bytes())
+            self.client_conn.send(PingFrame(flags=Frame.FLAG_ACK, payload=frame.payload).to_bytes())
             return
         self.log("Unexpected HTTP2 Frame: %s" % frame.human_readable(), "info")