# AUR0210

## Error Message

`AUR0210: Unrecognised collection type {{type}}.`

Where `{{type}}` is the runtime type description of the value Aurelia tried to treat as a collection.

## Description

This error occurs when Aurelia’s collection observation logic receives a value that it does not recognize as a supported collection type (for example not an `Array`, `Map`, `Set`, or another supported type in the given context).

## Common Scenarios

```html
<!-- ❌ items is an unexpected type -->
<template>
  <div repeat.for="item of items">${item}</div>
</template>
```

```ts
export class MyVm {
  // e.g. accidentally assigned an object instead of an array
  items: unknown = { a: 1 };
}
```

## Solutions

* Ensure the bound value is a supported collection type for the feature you’re using (most commonly an `Array`).
* Initialize collections to an empty array (`[]`) instead of `null/undefined` when appropriate.

## Troubleshooting

* Log the value being iterated/observed and confirm its type.
* Track where the value is assigned to locate the incorrect assignment.