diff --git a/src/doc-builders.js b/src/doc-builders.js
index 34c09e73..2dcb5f66 100644
--- a/src/doc-builders.js
+++ b/src/doc-builders.js
@@ -1,3 +1,5 @@
+"use strict";
+
 const assert = require("assert");
 const utils = require("./doc-utils");
 const hasHardLine = utils.hasHardLine;
diff --git a/src/doc-utils.js b/src/doc-utils.js
index f2837d64..b442d5d8 100644
--- a/src/doc-utils.js
+++ b/src/doc-utils.js
@@ -1,3 +1,4 @@
+"use strict";
 
 function iterDoc(topDoc, func) {
   const docs = [ topDoc ];