diff --git a/devtools/server/actors/webbrowser.js b/devtools/server/actors/webbrowser.js
index 933f342f56a0..6d7b7444b5f2 100644
--- a/devtools/server/actors/webbrowser.js
+++ b/devtools/server/actors/webbrowser.js
@@ -1089,6 +1089,10 @@ TabActor.prototype = {
   },
 
   onListWorkers: function BTA_onListWorkers(aRequest) {
+    if (!this.attached) {
+      return { error: "wrongState" };
+    }
+
     if (this._workerActorList === null) {
       this._workerActorList = new WorkerActorList({
         type: Ci.nsIWorkerDebugger.TYPE_DEDICATED,
diff --git a/devtools/server/actors/worker.js b/devtools/server/actors/worker.js
index 05b339ab292d..0be25e685a10 100644
--- a/devtools/server/actors/worker.js
+++ b/devtools/server/actors/worker.js
@@ -146,6 +146,7 @@ function WorkerActorList(options) {
 
 WorkerActorList.prototype = {
   getList: function () {
+    // Create a set of debuggers.
     let dbgs = new Set();
     let e = wdm.getWorkerDebuggerEnumerator();
     while (e.hasMoreElements()) {
@@ -155,12 +156,14 @@ WorkerActorList.prototype = {
       }
     }
 
+    // Delete each actor for which we don't have a debugger.
     for (let [dbg, ] of this._actors) {
       if (!dbgs.has(dbg)) {
         this._actors.delete(dbg);
       }
     }
 
+    // Create an actor for each debugger for which we don't have one.
     for (let dbg of dbgs) {
       if (!this._actors.has(dbg)) {
         this._actors.set(dbg, new WorkerActor(dbg));
@@ -172,8 +175,12 @@ WorkerActorList.prototype = {
       actors.push(actor);
     }
 
-    this._mustNotify = true;
-    this._checkListening();
+    if (!this._mustNotify) {
+      if (this._onListChanged !== null) {
+        wdm.addListener(this);
+      }
+      this._mustNotify = true;
+    }
 
     return Promise.resolve(actors);
   },
@@ -187,23 +194,24 @@ WorkerActorList.prototype = {
       throw new Error("onListChanged must be either a function or null.");
     }
 
-    this._onListChanged = onListChanged;
-    this._checkListening();
-  },
-
-  _checkListening: function () {
-    if (this._onListChanged !== null && this._mustNotify) {
-      wdm.addListener(this);
-    } else {
-      wdm.removeListener(this);
+    if (this._mustNotify) {
+      if (this._onListChanged === null && onListChanged !== null) {
+        wdm.addListener(this);
+      }
+      if (this._onListChanged !== null && onListChanged === null) {
+        wdm.removeListener(this);
+      }
     }
+    this._onListChanged = onListChanged;
   },
 
   _notifyListChanged: function () {
-    if (this._mustNotify) {
-      this._onListChanged();
-      this._mustNotify = false;
-    }
+     this._onListChanged();
+
+     if (this._onListChanged !== null) {
+       wdm.removeListener(this);
+     }
+     this._mustNotify = false;
   },
 
   onRegister: function (dbg) {