diff --git a/libavformat/nutenc.c b/libavformat/nutenc.c
index a92ff55c01..e9a3bb49db 100644
--- a/libavformat/nutenc.c
+++ b/libavformat/nutenc.c
@@ -1172,7 +1172,7 @@ static int nut_write_trailer(AVFormatContext *s)
 
     ret = avio_open_dyn_buf(&dyn_bc);
     if (ret >= 0 && nut->sp_count) {
-        av_assert1(nut->write_index);
+        av_assert1(nut->write_index); // sp_count should be 0 if no index is going to be written
         write_index(nut, dyn_bc);
         put_packet(nut, bc, dyn_bc, 1, INDEX_STARTCODE);
     }