diff --git a/sideline-load-cost.el b/sideline-load-cost.el
index 20c386f..577ab89 100644
--- a/sideline-load-cost.el
+++ b/sideline-load-cost.el
@@ -55,6 +55,11 @@
   "Face for load cost in sideline."
   :group 'sideline-load-cost)
 
+;;
+;;; Externals
+
+(defvar features)
+
 ;;
 ;;; Util
 
@@ -92,7 +97,7 @@ Optional argument EXT is use to drop-in replace the current extension."
   "Standard time use to measure average load time.")
 
 (defun sideline-load-cost--measure-load-time (size)
-  "Measure load time cost."
+  "Measure load time cost by file's SIZE."
   (* (/ (float size) sideline-load-cost--standard-file-size)
      (car sideline-load-cost--standard-time)))