# Unsupported transform in content config.

> **UnsupportedConfigTransformError**: `transform()` functions in your content config must return valid JSON, or data types compatible with the devalue library (including Dates, Maps, and Sets).\
> Full error: PARSE\_ERROR

## What went wrong?

[Section titled “What went wrong?”](#what-went-wrong)

`transform()` functions in your content config must return valid JSON, or data types compatible with the devalue library (including Dates, Maps, and Sets).

**See Also:**

* [devalue library](https://github.com/rich-harris/devalue)