diff --git a/dom/apps/UserCustomizations.jsm b/dom/apps/UserCustomizations.jsm
index e1fc4ac6a01c..89f10c1fd7d6 100644
--- a/dom/apps/UserCustomizations.jsm
+++ b/dom/apps/UserCustomizations.jsm
@@ -53,10 +53,16 @@ this.UserCustomizations = {
     });
 
     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) {