--- a/devtools/client/debugger/new/panel.js
+++ b/devtools/client/debugger/new/panel.js
@@ -44,18 +44,18 @@ DebuggerPanel.prototype = {
 
   _selectors: function() {
     return this.panelWin.Debugger.selectors;
   },
 
   getFrames: function() {
     let frames = this._selectors().getFrames(this._getState());
 
-    // frames is an empty array when the debugger is not paused
-    if (!frames.toJS) {
+    // frames is null when the debugger is not paused
+    if (!frames) {
       return {
         frames: [],
         selected: -1
       }
     }
 
     frames = frames.toJS();
     const selectedFrame = this._selectors().getSelectedFrame(this._getState());