--- a/dom/media/SeekTask.cpp
+++ b/dom/media/SeekTask.cpp
@@ -172,17 +172,20 @@ SeekJob&
 SeekTask::GetSeekJob()
 {
   return mSeekJob;
 }
 
 bool
 SeekTask::Exists() const
 {
-  return mSeekJob.Exists();
+  // mSeekTaskPromise communicates SeekTask and MDSM;
+  // mSeekJob communicates MDSM and MediaDecoder;
+  // Either one exists means the current seek task has yet finished.
+  return !mSeekTaskPromise.IsEmpty() || mSeekJob.Exists();
 }
 
 RefPtr<SeekTask::SeekTaskPromise>
 SeekTask::Seek(const media::TimeUnit& aDuration)
 {
   AssertOwnerThread();
 
   // Do the seek.