diff --git a/ctz_go1.9.go b/ctz_go1.9.go
index <HASH>..<HASH> 100644
--- a/ctz_go1.9.go
+++ b/ctz_go1.9.go
@@ -1,3 +1,5 @@
+// +build go1.9
+
 package roaring
 
 import "math/bits"
diff --git a/popcnt_go1.9.go b/popcnt_go1.9.go
index <HASH>..<HASH> 100644
--- a/popcnt_go1.9.go
+++ b/popcnt_go1.9.go
@@ -1,3 +1,5 @@
+// +build go1.9
+
 package roaring
 
 import "math/bits"
Attempt to fix build constraints for _go<I>.go files