diff --git a/libavformat/mux.c b/libavformat/mux.c
index 4edb6529c7..57044f3ab2 100644
--- a/libavformat/mux.c
+++ b/libavformat/mux.c
@@ -666,6 +666,8 @@ FF_DISABLE_DEPRECATION_WARNINGS
 FF_ENABLE_DEPRECATION_WARNINGS
 #endif
     pkt->buf       = NULL;
+    pkt->side_data = NULL;
+    pkt->side_data_elems = 0;
     if ((pkt->flags & AV_PKT_FLAG_UNCODED_FRAME)) {
         av_assert0(pkt->size == UNCODED_FRAME_PACKET_SIZE);
         av_assert0(((AVFrame *)pkt->data)->buf);