--- a/netwerk/cache2/CacheStorageService.cpp
+++ b/netwerk/cache2/CacheStorageService.cpp
@@ -2030,17 +2030,20 @@ CacheStorageService::GetCacheEntryInfo(C
                          fetchCount, lastModified, expirationTime,
                          aEntry->IsPinned(), info);
 }
 
 // static
 uint32_t CacheStorageService::CacheQueueSize(bool highPriority)
 {
   RefPtr<CacheIOThread> thread = CacheFileIOManager::IOThread();
-  MOZ_ASSERT(thread);
+  // The thread will be null at shutdown.
+  if (!thread) {
+    return 0;
+  }
   return thread->QueueSize(highPriority);
 }
 
 // Telemetry collection
 
 namespace {
 
 bool TelemetryEntryKey(CacheEntry const* entry, nsAutoCString& key)