diff --git a/libavcodec/internal.h b/libavcodec/internal.h
index 441430e41c..b435a359fb 100644
--- a/libavcodec/internal.h
+++ b/libavcodec/internal.h
@@ -120,6 +120,7 @@ int avpriv_unlock_avformat(void);
  *                If avpkt->data is already set, avpkt->size is checked
  *                to ensure it is large enough.
  *                If avpkt->data is NULL, a new buffer is allocated.
+ *                avpkt->size is set to the specified size.
  *                All other AVPacket fields will be reset with av_init_packet().
  * @param size    the minimum required packet size
  * @return        0 on success, negative error code on failure
diff --git a/libavcodec/utils.c b/libavcodec/utils.c
index 34eff0031a..606537b29a 100644
--- a/libavcodec/utils.c
+++ b/libavcodec/utils.c
@@ -839,16 +839,14 @@ int ff_alloc_packet(AVPacket *avpkt, int size)
 
     if (avpkt->data) {
         uint8_t *pkt_data;
-        int pkt_size;
 
         if (avpkt->size < size)
             return AVERROR(EINVAL);
 
         pkt_data = avpkt->data;
-        pkt_size = avpkt->size;
         av_init_packet(avpkt);
         avpkt->data = pkt_data;
-        avpkt->size = pkt_size;
+        avpkt->size = size;
         return 0;
     } else {
         return av_new_packet(avpkt, size);