--- a/dom/html/TextTrackManager.cpp
+++ b/dom/html/TextTrackManager.cpp
@@ -556,17 +556,17 @@ TextTrackManager::DispatchTimeMarchesOn(
 void
 TextTrackManager::TimeMarchesOn()
 {
   NS_ASSERTION(NS_IsMainThread(), "Wrong thread!");
 
   mTimeMarchesOnDispatched = false;
 
   // Early return if we don't have any TextTracks.
-  if (mTextTracks->Length() == 0) {
+  if (!mTextTracks || mTextTracks->Length() == 0) {
     return;
   }
 
   nsISupports* parentObject =
     mMediaElement->OwnerDoc()->GetParentObject();
   if (NS_WARN_IF(!parentObject)) {
     return;
   }