diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 2f07e11d87..13959f8b3d 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -1315,6 +1315,8 @@ static int ebml_parse(MatroskaDemuxContext *matroska,
                     matroska->num_levels--;
                     return LEVEL_ENDED;
                 }
+                // We have not encountered a known element; syntax is a sentinel.
+                av_assert1(syntax->type == EBML_NONE);
             };
         }