diff --git a/libavcodec/cbs.c b/libavcodec/cbs.c
index 0260ba6f67..47679eca1b 100644
--- a/libavcodec/cbs.c
+++ b/libavcodec/cbs.c
@@ -357,7 +357,8 @@ int ff_cbs_write_packet(CodedBitstreamContext *ctx,
     if (!buf)
         return AVERROR(ENOMEM);
 
-    av_init_packet(pkt);
+    av_buffer_unref(&pkt->buf);
+
     pkt->buf  = buf;
     pkt->data = frag->data;
     pkt->size = frag->data_size;
diff --git a/libavcodec/cbs.h b/libavcodec/cbs.h
index 967dcd1468..5260a39c63 100644
--- a/libavcodec/cbs.h
+++ b/libavcodec/cbs.h
@@ -297,7 +297,8 @@ int ff_cbs_write_fragment_data(CodedBitstreamContext *ctx,
 /**
  * Write the bitstream of a fragment to the extradata in codec parameters.
  *
- * This replaces any existing extradata in the structure.
+ * Modifies context and fragment as ff_cbs_write_fragment_data does and
+ * replaces any existing extradata in the structure.
  */
 int ff_cbs_write_extradata(CodedBitstreamContext *ctx,
                            AVCodecParameters *par,
@@ -305,6 +306,13 @@ int ff_cbs_write_extradata(CodedBitstreamContext *ctx,
 
 /**
  * Write the bitstream of a fragment to a packet.
+ *
+ * Modifies context and fragment as ff_cbs_write_fragment_data does.
+ *
+ * On success, the packet's buf is unreferenced and its buf, data and
+ * size fields are set to the corresponding values from the newly updated
+ * fragment; other fields are not touched.  On failure, the packet is not
+ * touched at all.
  */
 int ff_cbs_write_packet(CodedBitstreamContext *ctx,
                         AVPacket *pkt,