diff --git a/core/shared/src/main/scala/sigma/serialization/TypeSerializer.scala b/core/shared/src/main/scala/sigma/serialization/TypeSerializer.scala
index d22d4207b7..c21a3458c0 100644
--- a/core/shared/src/main/scala/sigma/serialization/TypeSerializer.scala
+++ b/core/shared/src/main/scala/sigma/serialization/TypeSerializer.scala
@@ -217,7 +217,8 @@ class TypeSerializer {
           SFunc(tDom, tRange, tpeParams)
         // todo: serialize tParams
         case _ =>
-          // todo: 6.0: replace 1008 check with identical but other opcode
+          // todo: 6.0: replace 1008 check with identical behavior but other opcode, to activate
+          //  ReplacedRule(1008 -> new opcode) during 6.0 activation
           CheckTypeCode(c.toByte)
           NoType
       }