diff --git a/src/modules/butterflies/butterflies_main.c b/src/modules/butterflies/butterflies_main.c
index c5c516bc2970d57e3fd4018f6077e100cc7e56ed..08acb3026047afc1646de7f80fe7cb038a75a14d 100644
--- a/src/modules/butterflies/butterflies_main.c
+++ b/src/modules/butterflies/butterflies_main.c
@@ -98,7 +98,7 @@ void load_butterflies_species_params(struct cats_configuration *conf, struct cat
                 check_and_create_directory_if_needed(path);
                 struct string_array *name = new_string_array_init("rw-debug");
                 string_array_add(name, conf->run_name);
-                data->debug_rw_filename = assemble_filename(path, name, "_", ".csv");
+                data->debug_rw_filename = assemble_filename(path, name, "_", "csv");
                 data->debug_rw_file = fopen(data->debug_rw_filename, "w");
                 ENSURE_FILE_OPENED(data->debug_rw_file, data->debug_rw_filename);
                 free_string_array(&path);