diff --git a/mitmproxy/protocol/http.py b/mitmproxy/protocol/http.py
index bdda80cd2..13556e9c9 100644
--- a/mitmproxy/protocol/http.py
+++ b/mitmproxy/protocol/http.py
@@ -184,7 +184,7 @@ class HttpLayer(Layer):
                 flow.request = request
                 # set upstream auth
                 if self.mode == "upstream" and self.config.upstream_auth is not None:
-                    self.data.headers["Proxy-Authorization"] = self.config.upstream_auth
+                    flow.request.headers["Proxy-Authorization"] = self.config.upstream_auth
                 self.process_request_hook(flow)
 
                 if not flow.response: