# AUR0199

## Error Message

`AUR0199: Trying to observe property {{property}} on null/undefined`

Where `{{property}}` is the property key Aurelia attempted to observe.

## Description

This error occurs when Aurelia’s observation system is asked to observe a property on a `null` or `undefined` object.

## Common Scenarios

```html
<!-- ❌ user is null/undefined -->
<template>
  <input value.bind="user.name">
</template>
```

```ts
export class MyVm {
  user: { name: string } | null = null;
}
```

## Solutions

* Initialize objects before binding to their properties.
* Guard bindings with `if.bind` until the object exists.

```html
<!-- ✅ Guard -->
<template>
  <div if.bind="user">
    <input value.bind="user.name">
  </div>
</template>
```

## Troubleshooting

* Check the stack trace to see which binding attempted to observe `{{property}}`.
* Log the binding context object to confirm it is not `null/undefined` at bind time.