diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index ec44d171a4..be176bcd6a 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -131,11 +131,7 @@ static void put_ebml_size(ByteIOContext *pb, uint64_t size, int bytes)
     int i, needed_bytes = ebml_size_bytes(size);
 
     // sizes larger than this are currently undefined in EBML
-    // so write "unknown" size
-    if (size >= (1ULL<<56)-1) {
-        put_ebml_size_unknown(pb, 1);
-        return;
-    }
+    assert(size < (1ULL<<56)-1);
 
     if (bytes == 0)
         // don't care how many bytes are used, so use the min