--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -490,17 +490,20 @@ public:
 
     if (!Reader()->ForceZeroStartTime()) {
       SLOG("Not Enough Data to seek at this stage, queuing seek");
       mMaster->mQueuedSeek.RejectIfExists(__func__);
       mMaster->mQueuedSeek.mTarget = aTarget;
       return mMaster->mQueuedSeek.mPromise.Ensure(__func__);
     }
 
-    mMaster->mQueuedSeek.RejectIfExists(__func__);
+    // Since ForceZeroStartTime() is true, we should've transitioned to SEEKING
+    // in Enter() if there is any queued seek.
+    MOZ_ASSERT(!mMaster->mQueuedSeek.Exists());
+
     SLOG("Changed state to SEEKING (to %lld)", aTarget.GetTime().ToMicroseconds());
     SeekJob seekJob;
     seekJob.mTarget = aTarget;
     return mMaster->InitiateSeek(Move(seekJob));
   }
 
 private:
   // Notify FirstFrameLoaded if having decoded first frames and