diff --git a/src/compwa_policy/utilities/pyproject/__init__.py b/src/compwa_policy/utilities/pyproject/__init__.py
index 4ba9a385..2f4a0a6d 100644
--- a/src/compwa_policy/utilities/pyproject/__init__.py
+++ b/src/compwa_policy/utilities/pyproject/__init__.py
@@ -106,7 +106,7 @@ def get_supported_python_versions(self) -> list[PythonVersion]:
         return get_supported_python_versions(self._document)
 
 
-@frozen
+@frozen(slots=False)
 class ModifiablePyproject(Pyproject, AbstractContextManager):
     """Stateful representation of a :code:`pyproject.toml` file.