diff --git a/sub/lavc_conv.c b/sub/lavc_conv.c
index 3dd6097d0f..73030207ef 100644
--- a/sub/lavc_conv.c
+++ b/sub/lavc_conv.c
@@ -79,11 +79,8 @@ struct lavc_conv *lavc_conv_create(struct mp_log *log, const char *codec_name,
     avctx = avcodec_alloc_context3(codec);
     if (!avctx)
         goto error;
-    avctx->extradata_size = extradata_len;
-    avctx->extradata = av_malloc(extradata_len);
-    if (!avctx->extradata)
+    if (mp_lavc_set_extradata(avctx, extradata, extradata_len) < 0)
         goto error;
-    memcpy(avctx->extradata, extradata, extradata_len);
     if (strcmp(codec_name, "eia_608") == 0)
         av_dict_set(&opts, "real_time", "1", 0);
     if (avcodec_open2(avctx, codec, &opts) < 0)