--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -4791,17 +4791,17 @@ HTMLMediaElement::UpdateReadyStateIntern
   // Now see if we should set HAVE_ENOUGH_DATA.
   // If it's something we don't know the size of, then we can't
   // make a real estimate, so we go straight to HAVE_ENOUGH_DATA once
   // we've downloaded enough data that our download rate is considered
   // reliable. We have to move to HAVE_ENOUGH_DATA at some point or
   // autoplay elements for live streams will never play. Otherwise we
   // move to HAVE_ENOUGH_DATA if we can play through the entire media
   // without stopping to buffer.
-  if (mDecoder->CanPlayThrough()) {
+  if (mWaitingForKey == NOT_WAITING_FOR_KEY && mDecoder->CanPlayThrough()) {
     LOG(LogLevel::Debug, ("MediaElement %p UpdateReadyStateInternal() "
                           "Decoder can play through", this));
     ChangeReadyState(nsIDOMHTMLMediaElement::HAVE_ENOUGH_DATA);
     return;
   }
   LOG(LogLevel::Debug, ("MediaElement %p UpdateReadyStateInternal() "
                         "Default; Decoder has future data", this));
   ChangeReadyState(nsIDOMHTMLMediaElement::HAVE_FUTURE_DATA);