diff --git a/libavcodec/movtextdec.c b/libavcodec/movtextdec.c
index 127ac6bd9d..cbaa6b2974 100644
--- a/libavcodec/movtextdec.c
+++ b/libavcodec/movtextdec.c
@@ -487,6 +487,7 @@ static int mov_text_decode_frame(AVCodecContext *avctx,
             //size_var is equal to 8 or 16 depending on the size of box
 
             if (tsmb_size == 0) {
+                av_log(avctx, AV_LOG_ERROR, "tsmb_size is 0\n");
                 return AVERROR_INVALIDDATA;
             }