diff --git a/mux_httpserver_test.go b/mux_httpserver_test.go
index 5d2f4d3..907ab91 100644
--- a/mux_httpserver_test.go
+++ b/mux_httpserver_test.go
@@ -1,3 +1,4 @@
+//go:build go1.9
 // +build go1.9
 
 package mux