diff --git a/sub/sd_ass.c b/sub/sd_ass.c
index 59336ff0ec..f8c9abebb2 100644
--- a/sub/sd_ass.c
+++ b/sub/sd_ass.c
@@ -609,7 +609,7 @@ static void fill_plaintext(struct sd *sd, double pts)
 static void reset(struct sd *sd)
 {
     struct sd_ass_priv *ctx = sd->priv;
-    if (sd->opts->sub_clear_on_seek) {
+    if (sd->opts->sub_clear_on_seek || ctx->duration_unknown) {
         ass_flush_events(ctx->ass_track);
         ctx->num_seen_packets = 0;
     }