diff --git a/src/command/config/schema.rs b/src/command/config/schema.rs
index 96c4a12..eaaf780 100644
--- a/src/command/config/schema.rs
+++ b/src/command/config/schema.rs
@@ -1,3 +1,8 @@
+// This module is only for the schema definition. Always use `Option<T>` for
+// optional values, and do not implement `Default`s or use `#[serde(default)]`s.
+// Optional values are handled in the main Firefish program, and this tool
+// does not take care of it.
+
 use serde::{Deserialize, Serialize};
 use validator::Validate;