diff --git a/src/savegame_json.cpp b/src/savegame_json.cpp
index 44d29450d294f..121994444654e 100644
--- a/src/savegame_json.cpp
+++ b/src/savegame_json.cpp
@@ -4629,8 +4629,8 @@ void field_type_migrations::load( const JsonObject &jo )
 {
     field_type_str_id from_field;
     field_type_str_id to_field;
-    mandatory( jo, true, "from_field", from_field );
-    mandatory( jo, true, "to_field", to_field );
+    mandatory( jo, false, "from_field", from_field );
+    mandatory( jo, false, "to_field", to_field );
     field_migrations.insert( std::make_pair( from_field, to_field ) );
 }
 
diff --git a/src/veh_type.cpp b/src/veh_type.cpp
index f8bdf2af72dae..47e7538fb70e3 100644
--- a/src/veh_type.cpp
+++ b/src/veh_type.cpp
@@ -1697,10 +1697,10 @@ void vpart_category::reset()
 void vpart_migration::load( const JsonObject &jo )
 {
     vpart_migration migration;
-    mandatory( jo, /* was_loaded = */ true, "from", migration.part_id_old );
-    mandatory( jo, /* was_loaded = */ true, "to", migration.part_id_new );
-    optional( jo, /* was_loaded = */ true, "variant", migration.variant );
-    optional( jo, /* was_loaded = */ true, "add_veh_tools", migration.add_veh_tools );
+    mandatory( jo, false, "from", migration.part_id_old );
+    mandatory( jo, false, "to", migration.part_id_new );
+    optional( jo, false, "variant", migration.variant );
+    optional( jo, false, "add_veh_tools", migration.add_veh_tools );
     vpart_migrations.emplace( migration.part_id_old, migration );
 }