# AUR0221 ## Error Message `AUR0221: Trying to assign value to readonly property "{{property}}" through computed observer.` Where `{{property}}` is the readonly property name. ## Description This error occurs when Aurelia tries to assign a value to a readonly property via a computed observer. Readonly properties (for example getters without setters) cannot be written to. ## Common Scenarios * Two-way binding to a getter-only property. * Binding a writable target to a computed/read-only source. ## Solution * Bind to writable properties for two-way scenarios. * If you need a computed-like value that is also assignable, implement a setter or use a separate backing field. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.aurelia.io/developer-guides/error-messages/0203-to-0227/aur0221.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.