# AUR0004 ### **Error message** Resolver for `yyyy` returned a null factory ### **Parameters** String version of the key being resolved ### Error explanation This error occurs when the DI container tries to resolve a transient registration, but no factory was found for the given key. A factory is required to create new instances for transients, and if it is missing or not properly registered, this error will be thrown. ### Common causes * You registered a transient dependency without providing a factory. * The factory for the key was not registered correctly or was accidentally removed. * There is a typo or mismatch in the key used for registration and resolution. * A plugin or library expected to register the factory did not do so. ### How to fix * Register a factory for the key using `container.registerFactory(IMyInterface, someFactoryObject)`. * Double-check that the key used for registration matches the key used for resolution. * Ensure that any plugins or libraries you are using are properly configured and registered. ### Debugging tips * Check the stack trace to see where the resolution was attempted. * Search your codebase for all registrations and resolutions of the key in question. * If using plugins, try disabling them one at a time to isolate the source. --- # 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/0001-to-0023/aur0004.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.