diff --git a/libavcodec/avpacket.c b/libavcodec/avpacket.c
index b3acc8838b..3480c23368 100644
--- a/libavcodec/avpacket.c
+++ b/libavcodec/avpacket.c
@@ -333,6 +333,8 @@ uint8_t *av_packet_get_side_data(AVPacket *pkt, enum AVPacketSideDataType type,
             return pkt->side_data[i].data;
         }
     }
+    if (size)
+        *size = 0;
     return NULL;
 }