> For the complete documentation index, see [llms.txt](https://docs.aurelia.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.aurelia.io/developer-guides/error-messages/4200-to-4206/aur4205.md).

# AUR4205

## Error Message

`AUR4205: Unknown expression of type {{type}}`

Where `{{type}}` is the runtime expression type encountered by the validation controller.

## Description

This error occurs when the validation controller encounters a binding/expression shape that it does not understand or cannot work with. This typically happens when using validation with an unsupported binding expression type or a custom/extended expression that the controller cannot interpret.

## Common Scenarios

* Using validation APIs against a binding that is not a standard property binding.
* Custom expression transforms/plugins that produce expression nodes the validation controller does not recognize.

## Solution

* Use validation with standard property bindings, or validate programmatically using a supported API.
* If you are extending expression parsing/compilation, ensure your extensions are compatible with validation-html.

## Troubleshooting

* Capture/log the expression type shown in `{{type}}` and locate where it originates.
* Reduce the binding to a simple `value.bind`/`checked.bind` and re-introduce complexity to find the incompatible part.


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