---
title: Fastly-IO-Error
summary: null
url: >-
  https://www.fastly.com/documentation/reference/http/http-headers/Fastly-IO-Error
---

This header is written by Fastly when a fatal error occurs in [Image Optimizer](https://docs.fastly.com/products/image-optimizer). The following error scenarios will prevent the image from being processed:

- Image exceeds maximum dimensions
- Image could not be parsed
- Not a supported image format
- Unsupported Content-Encoding
- Gzipped body exceeds maximum length
- Gzipped body could not be decoded
- Invalid status
- Response is pass
- Response is not cacheable