diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8f638a8..ae74097 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,5 +1,5 @@
 cmake_minimum_required(VERSION 3.25)
-project(image-gp-6 VERSION 0.0.37)
+project(image-gp-6 VERSION 0.0.38)
 
 include(FetchContent)
 
diff --git a/lib/blt-gp b/lib/blt-gp
index 552b150..6e45ea3 160000
--- a/lib/blt-gp
+++ b/lib/blt-gp
@@ -1 +1 @@
-Subproject commit 552b150402450c94d983b4df00191a65a947b806
+Subproject commit 6e45ea3d6de5c3686c969f9fdd706c527560cd8c
diff --git a/src/custom_transformer.cpp b/src/custom_transformer.cpp
index 52c3e8f..0a2a95e 100644
--- a/src/custom_transformer.cpp
+++ b/src/custom_transformer.cpp
@@ -332,7 +332,10 @@ namespace blt::gp
                     // other arguments are generated.
 
                     // get a replacement which returns the same type.
-                    operator_id random_replacement = program.get_random().select(program.get_type_non_terminals(current_func_info.return_type.id));
+                    auto& non_terminals = program.get_type_non_terminals(current_func_info.return_type.id);
+                    if (non_terminals.empty())
+                        continue;
+                    operator_id random_replacement = program.get_random().select(non_terminals);
                     blt::size_t arg_position = 0;
                     do
                     {
@@ -350,8 +353,11 @@ namespace blt::gp
                 exit:
                     auto& replacement_func_info = program.get_operator_info(random_replacement);
                     // replacement function should be valid. let's make a copy of us.
-                    blt::size_t for_bytes = 0;
-                    blt::size_t after_bytes = 0;
+                    auto current_end = c.find_endpoint(program, static_cast<blt::ptrdiff_t>(c_node));
+                    blt::size_t for_bytes = c.total_value_bytes(c_node, current_end);
+                    blt::size_t after_bytes = c.total_value_bytes(current_end);
+                    
+                    auto after_data =
                 }
                     break;
                 case mutation_operator::JUMP_FUNC: