diff --git a/eebie/writer.c b/eebie/writer.c
index 092b4ae..3cdd86d 100644
--- a/eebie/writer.c
+++ b/eebie/writer.c
@@ -32,7 +32,8 @@ static int bit_length(uint64_t i) {
 }
 
 static uint64_t encode_int(uint64_t i) {
-	return i | VARINT_MASKS[(bit_length(i) + 6) / 7];
+	// bit_length(i + 1) over bit_length(i) because 0xFF... lengths are interpreted as indefinite
+	return i | VARINT_MASKS[(bit_length(i + 1) + 6) / 7];
 }
 
 static void advance(EBMLWriter *this, uint64_t amount) {