diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index 3649ac25a2..0141fb0b8d 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -195,6 +195,8 @@ static void put_ebml_size_unknown(AVIOContext *pb, int bytes)
 {
     av_assert0(bytes <= 8);
     avio_w8(pb, 0x1ff >> bytes);
+    if (av_builtin_constant_p(bytes) && bytes == 1)
+        return;
     ffio_fill(pb, 0xff, bytes - 1);
 }