--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -1855,16 +1855,18 @@ void HTMLMediaElement::AbortExistingLoad
   mIsRunningSelectResource = false;
 
   if (mTextTrackManager) {
     mTextTrackManager->NotifyReset();
   }
 
   mEventDeliveryPaused = false;
   mPendingEvents.Clear();
+
+  AssertReadyStateIsNothing();
 }
 
 void HTMLMediaElement::NoSupportedMediaSourceError(const nsACString& aErrorDetails)
 {
   if (mDecoder) {
     ShutdownDecoder();
   }
   mErrorSink->SetError(MEDIA_ERR_SRC_NOT_SUPPORTED, aErrorDetails);
@@ -1916,16 +1918,18 @@ void HTMLMediaElement::QueueLoadFromSour
   }
 
   if (mDecoder) {
     // Reset readyState to HAVE_NOTHING since we're going to load a new decoder.
     ShutdownDecoder();
     ChangeReadyState(nsIDOMHTMLMediaElement::HAVE_NOTHING);
   }
 
+  AssertReadyStateIsNothing();
+
   ChangeDelayLoadStatus(true);
   ChangeNetworkState(nsIDOMHTMLMediaElement::NETWORK_LOADING);
   RefPtr<Runnable> r = NewRunnableMethod("HTMLMediaElement::LoadFromSourceChildren",
                                          this, &HTMLMediaElement::LoadFromSourceChildren);
   RunInStableState(r);
 }
 
 void HTMLMediaElement::QueueSelectResourceTask()
@@ -2574,16 +2578,18 @@ nsresult HTMLMediaElement::LoadResource(
       decoder->Shutdown();
       LOG(LogLevel::Debug,
           ("%p Failed to load for decoder %p", this, decoder.get()));
       return rv;
     }
     return FinishDecoderSetup(decoder);
   }
 
+  AssertReadyStateIsNothing();
+
   RefPtr<ChannelLoader> loader = new ChannelLoader;
   nsresult rv = loader->Load(this);
   if (NS_SUCCEEDED(rv)) {
     mChannelLoader = loader.forget();
   }
   return rv;
 }
 
@@ -4863,22 +4869,46 @@ HTMLMediaElement::CanPlayType(const nsAS
 
   LOG(LogLevel::Debug, ("%p CanPlayType(%s) = \"%s\"", this,
                      NS_ConvertUTF16toUTF8(aType).get(),
                      NS_ConvertUTF16toUTF8(aResult).get()));
 
   return NS_OK;
 }
 
+void
+HTMLMediaElement::AssertReadyStateIsNothing()
+{
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+  if (mReadyState != nsIDOMHTMLMediaElement::HAVE_NOTHING) {
+    char buf[1024];
+    SprintfLiteral(buf,
+                   "readyState=%d networkState=%d mLoadWaitStatus=%d "
+                   "mSourceLoadCandidate=%d "
+                   "mIsLoadingFromSourceChildren=%d mPreloadAction=%d "
+                   "mSuspendedForPreloadNone=%d error=%d",
+                   int(mReadyState.Ref()),
+                   int(mNetworkState),
+                   int(mLoadWaitStatus),
+                   !!mSourceLoadCandidate,
+                   mIsLoadingFromSourceChildren,
+                   int(mPreloadAction),
+                   mSuspendedForPreloadNone,
+                   GetError() ? GetError()->Code() : 0);
+    MOZ_CRASH_UNSAFE_PRINTF("ReadyState should be HAVE_NOTHING! %s", buf);
+  }
+#endif
+}
+
 nsresult
 HTMLMediaElement::InitializeDecoderAsClone(ChannelMediaDecoder* aOriginal)
 {
   NS_ASSERTION(mLoadingSrc, "mLoadingSrc must already be set");
   NS_ASSERTION(mDecoder == nullptr, "Shouldn't have a decoder");
-  MOZ_DIAGNOSTIC_ASSERT(mReadyState == nsIDOMHTMLMediaElement::HAVE_NOTHING);
+  AssertReadyStateIsNothing();
 
   MediaDecoderInit decoderInit(this,
                                mMuted ? 0.0 : mVolume,
                                mPreservesPitch,
                                mPlaybackRate,
                                mPreloadAction ==
                                  HTMLMediaElement::PRELOAD_METADATA,
                                mHasSuspendTaint,
@@ -4922,17 +4952,17 @@ HTMLMediaElement::SetupDecoder(DecoderTy
 
   return rv;
 }
 
 nsresult HTMLMediaElement::InitializeDecoderForChannel(nsIChannel* aChannel,
                                                        nsIStreamListener** aListener)
 {
   NS_ASSERTION(mLoadingSrc, "mLoadingSrc must already be set");
-  MOZ_DIAGNOSTIC_ASSERT(mReadyState == nsIDOMHTMLMediaElement::HAVE_NOTHING);
+  AssertReadyStateIsNothing();
 
   DecoderDoctorDiagnostics diagnostics;
 
   nsAutoCString mimeType;
   aChannel->GetContentType(mimeType);
   NS_ASSERTION(!mimeType.IsEmpty(), "We should have the Content-Type.");
   NS_ConvertUTF8toUTF16 mimeUTF16(mimeType);
 
@@ -6581,16 +6611,17 @@ void HTMLMediaElement::NotifyAddedSource
 {
   // If a source element is inserted as a child of a media element
   // that has no src attribute and whose networkState has the value
   // NETWORK_EMPTY, the user agent must invoke the media element's
   // resource selection algorithm.
   if (!HasAttr(kNameSpaceID_None, nsGkAtoms::src) &&
       mNetworkState == nsIDOMHTMLMediaElement::NETWORK_EMPTY)
   {
+    AssertReadyStateIsNothing();
     QueueSelectResourceTask();
   }
 
   // A load was paused in the resource selection algorithm, waiting for
   // a new source child to be added, resume the resource selection algorithm.
   if (mLoadWaitStatus == WAITING_FOR_SOURCE) {
     // Rest the flag so we don't queue multiple LoadFromSourceTask() when
     // multiple <source> are attached in an event loop.

--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -1815,16 +1815,19 @@ private:
   // We keep track of these because the load algorithm resolves/rejects all
   // already-dispatched pending play promises.
   nsTArray<nsResolveOrRejectPendingPlayPromisesRunner*> mPendingPlayPromisesRunners;
 
   // A pending seek promise which is created at Seek() method call and is
   // resolved/rejected at AsyncResolveSeekDOMPromiseIfExists()/
   // AsyncRejectSeekDOMPromiseIfExists() methods.
   RefPtr<dom::Promise> mSeekDOMPromise;
+
+  // For debugging bug 1407148.
+  void AssertReadyStateIsNothing();
 };
 
 // Check if the context is chrome or has the debugger or tabs permission
 bool
 HasDebuggerOrTabsPrivilege(JSContext* aCx, JSObject* aObj);
 
 } // namespace dom
 } // namespace mozilla