diff --git a/src/cmd/go/internal/modload/testgo.go b/src/cmd/go/internal/modload/testgo.go
index 6cfba0c68fc..663b24a68d7 100644
--- a/src/cmd/go/internal/modload/testgo.go
+++ b/src/cmd/go/internal/modload/testgo.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//+build testgo
+
 package modload
 
 func init() {