diff --git a/xpcom/threads/ThreadEventTarget.cpp b/xpcom/threads/ThreadEventTarget.cpp
index cb229669777c..695e62a06b75 100644
--- a/xpcom/threads/ThreadEventTarget.cpp
+++ b/xpcom/threads/ThreadEventTarget.cpp
@@ -23,7 +23,12 @@ using namespace mozilla;
 
 ThreadEventTarget::ThreadEventTarget(ThreadTargetSink* aSink,
                                      bool aIsMainThread)
-    : mSink(aSink), mIsMainThread(aIsMainThread) {
+    : mSink(aSink)
+#ifdef DEBUG
+      ,
+      mIsMainThread(aIsMainThread)
+#endif
+{
   mThread = PR_GetCurrentThread();
 }
 
@@ -52,10 +57,9 @@ ThreadEventTarget::Dispatch(already_AddRefed<nsIRunnable> aEvent,
     return NS_ERROR_INVALID_ARG;
   }
 
-  if (gXPCOMThreadsShutDown && !mIsMainThread) {
-    NS_ASSERTION(false, "Failed Dispatch after xpcom-shutdown-threads");
-    return NS_ERROR_ILLEGAL_DURING_SHUTDOWN;
-  }
+  NS_ASSERTION(!gXPCOMThreadsShutDown || mIsMainThread ||
+                   PR_GetCurrentThread() == mThread,
+               "Dispatch to non-main thread after xpcom-shutdown-threads");
 
   LogRunnable::LogDispatch(event.get());
 
diff --git a/xpcom/threads/ThreadEventTarget.h b/xpcom/threads/ThreadEventTarget.h
index 6ced779fb987..5b5b188fdaf3 100644
--- a/xpcom/threads/ThreadEventTarget.h
+++ b/xpcom/threads/ThreadEventTarget.h
@@ -47,7 +47,9 @@ class ThreadEventTarget final : public nsISerialEventTarget {
   ~ThreadEventTarget();
 
   RefPtr<ThreadTargetSink> mSink;
+#ifdef DEBUG
   bool mIsMainThread;
+#endif
 };
 
 }  // namespace mozilla