--- a/toolkit/components/thumbnails/BackgroundPageThumbs.jsm
+++ b/toolkit/components/thumbnails/BackgroundPageThumbs.jsm
@@ -394,17 +394,17 @@ Object.defineProperty(this, "BackgroundP
  */
 function Capture(url, captureCallback, options) {
   this.url = url;
   this.captureCallback = captureCallback;
   this.options = options;
   this.id = Capture.nextID++;
   this.creationDate = new Date();
   this.doneCallbacks = [];
-  this.doneReason;
+  this.doneReason = -1;
   if (options.onDone)
     this.doneCallbacks.push(options.onDone);
 }
 
 Capture.prototype = {
 
   get pending() {
     return !!this._msgMan;