--- a/devtools/server/actors/inspector.js
+++ b/devtools/server/actors/inspector.js
@@ -228,16 +228,19 @@ var NodeActor = exports.NodeActor = prot
 
   isDocumentElement: function () {
     return this.rawNode.ownerDocument &&
            this.rawNode.ownerDocument.documentElement === this.rawNode;
   },
 
   destroy: function () {
     protocol.Actor.prototype.destroy.call(this);
+    if (!this.walker) {
+      return;
+    }
 
     if (this.mutationObserver) {
       if (!Cu.isDeadWrapper(this.mutationObserver)) {
         this.mutationObserver.disconnect();
       }
       this.mutationObserver = null;
     }
     this.rawNode = null;