diff --git a/dom/workers/WorkerRunnable.cpp b/dom/workers/WorkerRunnable.cpp
index 123455f0475e..ba641ed38995 100644
--- a/dom/workers/WorkerRunnable.cpp
+++ b/dom/workers/WorkerRunnable.cpp
@@ -260,6 +260,10 @@ WorkerRunnable::Run()
 
     MOZ_ASSERT(IsCanceled(), "Subclass Cancel() didn't set IsCanceled()!");
 
+    if (mBehavior == WorkerThreadModifyBusyCount) {
+      mWorkerPrivate->ModifyBusyCountFromWorker(false);
+    }
+
     return NS_OK;
   }