> 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/0001-to-0023/aur0019.md).

# AUR0019

Invalid channel name or type: yyyy.

### **Error message**

Invalid channel name or type: yyyy.

### **Parameters**

Channel name or type (string or constructor)

### Error explanation

This error occurs when you attempt to subscribe to an event in the EventAggregator using an invalid channel name (such as `undefined`, `null`, or an invalid type). The EventAggregator expects a valid string channel name or a constructor function for message types.

### Common causes

* Passing `undefined`, `null`, or an invalid value as the channel or type to `subscribe()` or `subscribeOnce()`.
* Typo or incorrect variable used for the channel name or type.
* Attempting to subscribe before the channel or type is defined or available.

### How to fix

* Ensure you are passing a valid string or constructor as the channel or type when subscribing.
* Double-check for typos or incorrect variables in your subscription code.
* Make sure the channel or type is defined and available before subscribing.

### Debugging tips

* Check the stack trace to see where the invalid subscription was attempted.
* Search your codebase for all calls to `subscribe()` and `subscribeOnce()` in the EventAggregator.
* If using plugins, try disabling them one at a time to isolate the source.


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