--- a/dom/apps/UserCustomizations.jsm
+++ b/dom/apps/UserCustomizations.jsm
@@ -48,20 +48,26 @@ this.UserCustomizations = {
     debug("Starting customization registration for " + aApp.manifestURL + "\n");
 
     let extension = new Extension({
       id: AppsUtils.computeHash(aApp.manifestURL),
       resourceURI: Services.io.newURI(aApp.origin + "/", null, null)
     });
 
     this.extensions.set(aApp.manifestURL, extension);
-    extension.startup().then(() => {
-      let uri = Services.io.newURI(aApp.origin, null, null);
-      this.appId.add(uri.host);
-    });
+    let uri = Services.io.newURI(aApp.origin, null, null);
+    debug(`Adding ${uri.host} to appId set`);
+    this.appId.add(uri.host);
+
+    extension.startup()
+      .then(() => { })
+      .catch((err) => {
+        debug(`extension.startup failed: ${err}`);
+        this.appId.delete(uri.host);
+      });
   },
 
   unregister: function(aApp) {
     if (!this._enabled) {
       return;
     }
 
     debug("Starting customization unregistration for " + aApp.manifestURL);