diff --git a/tools/measure.go b/tools/measure.go
index 2a11ae7..9ea4e10 100644
--- a/tools/measure.go
+++ b/tools/measure.go
@@ -7,6 +7,7 @@ import (
     "path/filepath"
     "regexp"
     "strings"
+    "unicode/utf8"
 )
 
 func check(err error) {