diff --git a/pkgs/by-name/ha/harlequin/package.nix b/pkgs/by-name/ha/harlequin/package.nix
index b12abb6b7a03..186fa4ae2c04 100644
--- a/pkgs/by-name/ha/harlequin/package.nix
+++ b/pkgs/by-name/ha/harlequin/package.nix
@@ -28,7 +28,7 @@ let
       });
 
       textual-textarea = super.textual-textarea.overridePythonAttrs (old: {
-        pythonRelaxDeps = old.pythonRelaxDeps ++ [ "textual" ];
+        pythonRelaxDeps = (old.pythonRelaxDeps or [ ]) ++ [ "textual" ];
       });
     };
   };