diff --git a/src/lib_ccx/matroska.c b/src/lib_ccx/matroska.c
index 3b3d8a815..3da24e991 100644
--- a/src/lib_ccx/matroska.c
+++ b/src/lib_ccx/matroska.c
@@ -1333,6 +1333,8 @@ char *generate_filename_from_track(struct matroska_ctx *mkv_ctx, struct matroska
 {
 	const char *basename = get_basename(mkv_ctx->filename);
 	const char *extension = matroska_track_text_subtitle_id_extensions[track->codec_id];
+	if (extension == NULL)
+		extension = "bin";
 	/* Prefer the BCP-47 IETF tag (e.g. "zh-Hant") over the legacy
 	 * ISO-639-2 code (e.g. "chi") when one is available. */
 	const char *lang_tag = track->lang_ietf ? track->lang_ietf : track->lang;
diff --git a/src/lib_ccx/matroska.h b/src/lib_ccx/matroska.h
index d5ebcd64b..8493cb3b6 100644
--- a/src/lib_ccx/matroska.h
+++ b/src/lib_ccx/matroska.h
@@ -179,7 +179,7 @@ char *matroska_track_text_subtitle_id_strings[] = {
 char *matroska_track_text_subtitle_id_extensions[] = {
     "srt", "ssa", "ass",
     "usf", "vtt", "bmp",
-    "idx", NULL // Unknown
+    "idx", "kate", NULL // Unknown/sentinel
 };
 
 char *avc_codec_id = "V_MPEG4/ISO/AVC";