--- a/toolkit/mozapps/extensions/test/xpcshell/test_provider_shutdown.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_provider_shutdown.js
@@ -53,16 +53,18 @@ function findInStatus(aStatus, aName) {
 
 /*
  * Make sure we report correctly when an add-on provider or AddonRepository block shutdown
  */
 add_task(function* blockRepoShutdown() {
   // Reach into the AddonManager scope and inject our mock AddonRepository
   // the mock provider behaves enough like AddonRepository for the purpose of this test
   let mockRepo = mockAddonProvider("Mock repo");
+  // Trigger the lazy getter so that we can assign a new value to it:
+  void AMscope.AddonRepository;
   AMscope.AddonRepository = mockRepo;
 
   let mockProvider = mockAddonProvider("Mock provider");
 
   startupManager();
   AddonManagerPrivate.registerProvider(mockProvider);
 
   // Start shutting the manager down