diff --git a/dom/asmjscache/AsmJSCache.cpp b/dom/asmjscache/AsmJSCache.cpp
index a97175a1039f..52468fb4fdab 100644
--- a/dom/asmjscache/AsmJSCache.cpp
+++ b/dom/asmjscache/AsmJSCache.cpp
@@ -1938,6 +1938,9 @@ void
 Client::ShutdownWorkThreads()
 {
   AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mShutdownRequested);
+
+  mShutdownRequested = true;
 
   if (sLiveParentActors) {
     MOZ_ALWAYS_TRUE(SpinEventLoopUntil([&]() {