diff --git a/dom/media/MediaDecoder.cpp b/dom/media/MediaDecoder.cpp
index bb21795ced71..2a96be23d12b 100644
--- a/dom/media/MediaDecoder.cpp
+++ b/dom/media/MediaDecoder.cpp
@@ -416,6 +416,8 @@ MediaDecoder::MediaDecoder(MediaDecoderOwner* aOwner)
   , INIT_CANONICAL(mDecoderPosition, 0)
   , INIT_CANONICAL(mIsVisible, !aOwner->IsHidden())
   , mTelemetryReported(false)
+  , mIsMediaElement(!!aOwner->GetMediaElement())
+  , mElement(aOwner->GetMediaElement())
 {
   MOZ_ASSERT(NS_IsMainThread());
   MediaMemoryTracker::AddMediaDecoder(this);
@@ -1650,6 +1652,8 @@ MediaDecoderOwner*
 MediaDecoder::GetOwner() const
 {
   MOZ_ASSERT(NS_IsMainThread());
+  // Check object lifetime when mOwner points to a media element.
+  MOZ_DIAGNOSTIC_ASSERT(!mOwner || !mIsMediaElement || mElement);
   // mOwner is valid until shutdown.
   return mOwner;
 }
diff --git a/dom/media/MediaDecoder.h b/dom/media/MediaDecoder.h
index 487b96687d7d..16d3e1138120 100644
--- a/dom/media/MediaDecoder.h
+++ b/dom/media/MediaDecoder.h
@@ -42,6 +42,7 @@ namespace mozilla {
 
 namespace dom {
 class Promise;
+class HTMLMediaElement;
 }
 
 class VideoFrameContainer;
@@ -845,6 +846,10 @@ private:
   void NotifyDownloadEnded(nsresult aStatus);
 
   bool mTelemetryReported;
+
+  // Used to debug how mOwner becomes a dangling pointer in bug 1326294.
+  bool mIsMediaElement;
+  WeakPtr<dom::HTMLMediaElement> mElement;
 };
 
 } // namespace mozilla