diff --git a/modules/index.js b/modules/index.js
index 17678d491..a6d5c4f8d 100644
--- a/modules/index.js
+++ b/modules/index.js
@@ -19,9 +19,11 @@ export * from './ui/index';
 export * from './util/index';
 export * from './validations/index';
 
+// When `debug = true`, we use `Object.freeze` on immutables in iD.
+// This is only done in testing because of the performance penalty.
 export let debug = false;
 
-// reexport just what our tests use, see #4379
+// Reexport just what our tests use, see #4379
 import * as D3 from 'd3';
 export let d3 = {
   customEvent: D3.customEvent,