diff --git a/ext/libav/gstavaudenc.c b/ext/libav/gstavaudenc.c
index a9aa88ad35b4f839f4dda5530138caa9ad97e29c..04081049aa3850c5100f77d5f58cd7c98ad74545 100644
--- a/ext/libav/gstavaudenc.c
+++ b/ext/libav/gstavaudenc.c
@@ -554,6 +554,9 @@ gst_ffmpegaudenc_encode_audio (GstFFMpegAudEnc * ffmpegaudenc,
 
     codec = ffmpegaudenc->context->codec;
     if ((codec->capabilities & CODEC_CAP_VARIABLE_FRAME_SIZE) || !audio_in) {
+      /* FIXME: Not really correct, as -1 means "all the samples we got
+         given so far", which may not be true depending on the codec,
+         but we have no way to know AFAICT */
       ret = gst_audio_encoder_finish_frame (enc, outbuf, -1);
     } else {
       ret = gst_audio_encoder_finish_frame (enc, outbuf, frame.nb_samples);