diff --git a/dom/workers/WorkerPrivate.cpp b/dom/workers/WorkerPrivate.cpp
index 85bcc2ffef..fb2e4d7b56 100644
--- a/dom/workers/WorkerPrivate.cpp
+++ b/dom/workers/WorkerPrivate.cpp
@@ -5161,7 +5161,9 @@ WorkerPrivate::ScheduleDeletion(WorkerRanOrNot aRanOrNot)
   if (WorkerRan == aRanOrNot) {
     nsIThread* currentThread = NS_GetCurrentThread();
     MOZ_ASSERT(currentThread);
-    MOZ_ASSERT(!NS_HasPendingEvents(currentThread));
+    // On the worker thread WorkerRunnable will refuse to run if not nested
+    // on top of a WorkerThreadPrimaryRunnable.
+    Unused << NS_WARN_IF(NS_HasPendingEvents(currentThread));
   }
 #endif
 
diff --git a/dom/workers/WorkerRunnable.cpp b/dom/workers/WorkerRunnable.cpp
index be0171b768..eb56650b51 100644
--- a/dom/workers/WorkerRunnable.cpp
+++ b/dom/workers/WorkerRunnable.cpp
@@ -11,6 +11,7 @@
 #include "nsIRunnable.h"
 #include "nsThreadUtils.h"
 
+#include "mozilla/CycleCollectedJSContext.h"
 #include "mozilla/DebugOnly.h"
 #include "mozilla/ErrorResult.h"
 #include "mozilla/Unused.h"
@@ -231,6 +232,19 @@ WorkerRunnable::Run()
   bool targetIsWorkerThread = mBehavior == WorkerThreadModifyBusyCount ||
                               mBehavior == WorkerThreadUnchangedBusyCount;
 
+  if (targetIsWorkerThread) {
+    // On a worker thread, a WorkerRunnable should only run when there is an
+    // underlying WorkerThreadPrimaryRunnable active, which means we should
+    // find a CycleCollectedJSContext.
+    if (!CycleCollectedJSContext::Get()) {
+      MOZ_DIAGNOSTIC_ASSERT(false,
+                            "A WorkerRunnable was executed after "
+                            "WorkerThreadPrimaryRunnable ended.");
+
+      return NS_OK;
+    }
+  }
+
 #ifdef DEBUG
   MOZ_ASSERT_IF(mCallingCancelWithinRun, targetIsWorkerThread);
   if (targetIsWorkerThread) {