diff --git a/modules/rtp_rtcp/source/rtp_sender_video.cc b/modules/rtp_rtcp/source/rtp_sender_video.cc
index 2ca5e4bb1e..fc39225d7e 100644
--- a/modules/rtp_rtcp/source/rtp_sender_video.cc
+++ b/modules/rtp_rtcp/source/rtp_sender_video.cc
@@ -566,6 +566,8 @@ bool RTPSenderVideo::SendVideo(int payload_type,
     // Disable attaching dependency descriptor to delta packets (including
     // non-first packet of a key frame) when it wasn't attached to a key frame,
     // as dependency descriptor can't be usable in such case.
+    // This can also happen when the descriptor is larger than 15 bytes and
+    // two-byte header extensions are not negotiated using extmap-allow-mixed.
     RTC_LOG(LS_WARNING) << "Disable dependency descriptor because failed to "
                            "attach it to a key frame.";
     video_structure_ = nullptr;