diff --git a/lib/tun2/server.go b/lib/tun2/server.go
index 9b2fb80..74694a5 100644
--- a/lib/tun2/server.go
+++ b/lib/tun2/server.go
@@ -472,7 +472,7 @@ func (s *Server) RoundTrip(req *http.Request) (*http.Response, error) {
 				ProtoMajor:    req.ProtoMajor,
 				ProtoMinor:    req.ProtoMinor,
 				Header:        reshdr,
-				ContentLength: len(resbody),
+				ContentLength: int64(len(resbody)),
 				Close:         true,
 				Request:       req,
 			}