diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 385c2c3..0a50328 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -4434,6 +4434,11 @@
     cm->reset_decoder_state = 0;
 
     if (cm->show_existing_frame) {
+      if (pbi->sequence_header_changed) {
+        aom_internal_error(
+            &cm->error, AOM_CODEC_CORRUPT_FRAME,
+            "New sequence header starts with a show_existing_frame.");
+      }
       // Show an existing frame directly.
       const int existing_frame_idx = aom_rb_read_literal(rb, 3);
       const int frame_to_show = cm->ref_frame_map[existing_frame_idx];