diff --git a/src/net/http/serve_test.go b/src/net/http/serve_test.go
index 95e6bf4adb..f8687416fe 100644
--- a/src/net/http/serve_test.go
+++ b/src/net/http/serve_test.go
@@ -6460,13 +6460,15 @@ func TestDisableKeepAliveUpgrade(t *testing.T) {
 		w.Header().Set("Connection", "Upgrade")
 		w.Header().Set("Upgrade", "someProto")
 		w.WriteHeader(StatusSwitchingProtocols)
-		c, _, err := w.(Hijacker).Hijack()
+		c, buf, err := w.(Hijacker).Hijack()
 		if err != nil {
 			return
 		}
 		defer c.Close()
 
-		io.Copy(c, c)
+		// Copy from the *bufio.ReadWriter, which may contain buffered data.
+		// Copy to the net.Conn, to avoid buffering the output.
+		io.Copy(c, buf)
 	}))
 	s.Config.SetKeepAlivesEnabled(false)
 	s.Start()