Maarten de Coen focused on backend stability and data integrity improvements in the columnflow/columnflow repository, addressing two nuanced bugs over a one-month period. He refactored the axis validation logic in the CreateHistograms module, ensuring that empty axes were no longer misclassified as missing by distinguishing between None and empty lists. Additionally, he corrected the stats output format from .parquet to .json, aligning the export with downstream JSON handling requirements. Working primarily in Python, Maarten demonstrated careful attention to edge-case validation, file I/O normalization, and clear commit documentation, resulting in more reliable histogram generation and consistent data exports.