diff --git a/.eslintrc b/.eslintrc
index fa7fcb81..8f853407 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -22,6 +22,7 @@
     "dot-notation": [2, { "allowKeywords": false }],
     "eol-last": 2,
     "eqeqeq": [2, "smart"],
+    "guard-for-in": 2,
     "indent": [2, 2, {"SwitchCase": 1}],
     "key-spacing": 1,
     "linebreak-style": 2,
diff --git a/js/main-window-dom.js b/js/main-window-dom.js
index dd50660e..35df3654 100644
--- a/js/main-window-dom.js
+++ b/js/main-window-dom.js
@@ -7,7 +7,9 @@ function el(tagName, attributes, content) {
     content = [content];
   }
   for (var attr in attributes) {
-    tag.setAttribute(attr, attributes[attr]);
+    if (attributes.hasOwnProperty(attr)) {
+      tag.setAttribute(attr, attributes[attr]);
+    }
   }
   for (var i = 0; i < content.length; i++) {
     if (typeof content[i] === 'string') {
diff --git a/js/multi-display.js b/js/multi-display.js
index 21a2ac77..4f27ff6a 100644
--- a/js/multi-display.js
+++ b/js/multi-display.js
@@ -27,8 +27,10 @@ MultiDisplay.prototype.setRegisterText = function(register, text) {
 
 MultiDisplay.prototype.showRegister = function(showKey) {
   for (var key in this.registers) {
-    var display = (key === showKey) ? 'block' : 'none';
-    this.registers[key].node.style.display = display;
+    if (this.registers.hasOwnProperty(key)) {
+      var display = (key === showKey) ? 'block' : 'none';
+      this.registers[key].node.style.display = display;
+    }
   }
 };