diff --git a/pkgs/pkgs-lib/formats.nix b/pkgs/pkgs-lib/formats.nix
index 25d13d367d99..740d3f783834 100644
--- a/pkgs/pkgs-lib/formats.nix
+++ b/pkgs/pkgs-lib/formats.nix
@@ -63,6 +63,7 @@ let
 
     Parameters:
     - typeName: String describing the format (e.g. "JSON", "YAML", "XML")
+    - nullable: Whether the structured value type allows `null` values.
 
     Returns a type suitable for structured data formats that supports:
     - Basic types: boolean, integer, float, string, path