diff --git a/halo2-base/src/safe_types/mod.rs b/halo2-base/src/safe_types/mod.rs
index dbcd3636..205c314e 100644
--- a/halo2-base/src/safe_types/mod.rs
+++ b/halo2-base/src/safe_types/mod.rs
@@ -324,6 +324,7 @@ impl<'a, F: ScalarField> SafeTypeChip<'a, F> {
         )
     }
 
+    /// Assumes that `bits <= inputs.len() * 8`.
     fn add_bytes_constraints(
         &self,
         ctx: &mut Context<F>,