diff --git a/js/iD/graph/Entity.js b/js/iD/graph/Entity.js
index acbdb1dd6..ff3208473 100644
--- a/js/iD/graph/Entity.js
+++ b/js/iD/graph/Entity.js
@@ -1,11 +1,16 @@
 iD.Entity = function(a, b) {
     if (!(this instanceof iD.Entity)) return new iD.Entity(a, b);
 
-    _.extend(this, a, b);
+    _.extend(this, {tags: {}}, a, b);
 
     if (b) {
         this.modified = true;
     }
+
+    if (iD.debug) {
+        Object.freeze(this);
+        Object.freeze(this.tags);
+    }
 };
 
 iD.Entity.prototype = {
diff --git a/js/iD/graph/Graph.js b/js/iD/graph/Graph.js
index ef492a005..1708218ec 100644
--- a/js/iD/graph/Graph.js
+++ b/js/iD/graph/Graph.js
@@ -1,6 +1,9 @@
 iD.Graph = function(entities, annotation) {
     if (!(this instanceof iD.Graph)) return new iD.Graph(entities, annotation);
+
     this.entities = entities || {};
+    this.annotation = annotation;
+
     for (var id in this.entities) {
         // TODO: update extents that need it.
         if (this.entities[id].type === 'way' && !this.entities[id]._extent) {
@@ -16,7 +19,11 @@ iD.Graph = function(entities, annotation) {
             this.entities[id]._extent = extent;
         }
     }
-    this.annotation = annotation;
+
+    if (iD.debug) {
+        Object.freeze(this);
+        Object.freeze(this.entities);
+    }
 };
 
 iD.Graph.prototype = {
diff --git a/test/index.html b/test/index.html
index 01f6316ef..33d842123 100644
--- a/test/index.html
+++ b/test/index.html
@@ -53,6 +53,10 @@
   <script type="text/javascript" src="spec/XML.js"></script>
   <script type="text/javascript" src="spec/Inspector.js"></script>
 
+  <script type="text/javascript">
+    iD.debug = true;
+  </script>
+
   <script type="text/javascript">
     (function() {
       var jasmineEnv = jasmine.getEnv();