--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -1872,17 +1872,18 @@ NS_IMETHODIMP HTMLMediaElement::GetDurat
   *aDuration = Duration();
   return NS_OK;
 }
 
 already_AddRefed<TimeRanges>
 HTMLMediaElement::Seekable() const
 {
   RefPtr<TimeRanges> ranges = new TimeRanges(ToSupports(OwnerDoc()));
-  if (mDecoder && mReadyState > nsIDOMHTMLMediaElement::HAVE_NOTHING) {
+  if (mMediaSource ||
+      (mDecoder && mReadyState > nsIDOMHTMLMediaElement::HAVE_NOTHING)) {
     mDecoder->GetSeekable().ToTimeRanges(ranges);
   }
   return ranges.forget();
 }
 
 NS_IMETHODIMP HTMLMediaElement::GetSeekable(nsIDOMTimeRanges** aSeekable)
 {
   RefPtr<TimeRanges> ranges = Seekable();