diff --git a/libavcodec/proresdec.c b/libavcodec/proresdec.c
index 00eb38d1c6..fc7a55bfc8 100644
--- a/libavcodec/proresdec.c
+++ b/libavcodec/proresdec.c
@@ -480,6 +480,7 @@ static int decode_slice_thread(AVCodecContext *avctx, void *arg, int jobnr, int
     y_data_size = AV_RB16(buf + 2);
     u_data_size = AV_RB16(buf + 4);
     v_data_size = slice->data_size - y_data_size - u_data_size - hdr_size;
+    if (hdr_size > 7) v_data_size = AV_RB16(buf + 6);
 
     if (y_data_size < 0 || u_data_size < 0 || v_data_size < 0) {
         av_log(avctx, AV_LOG_ERROR, "invalid plane data size\n");