angular/aio/content/errors/NG01203.md

@name Missing value accessor @category runtime @shortDescription You must register an NgValueAccessor with a custom form control

@description For all custom form controls, you must register a value accessor.

Here's an example of how to provide one:

providers: [
  {
    provide: NG_VALUE_ACCESSOR,
    useExisting: forwardRef(() => MyInputField),
    multi: true,
  }
]

@debugging As described above, your control was expected to have a value accessor, but was missing one. However, there are many different reasons this can happen in practice. Here's a listing of some known problems leading to this error.

1. If you defined a custom form control, did you remember to provide a value accessor?
2. Did you put ngModel on an element with no value, or an invalid element (e.g. <div [(ngModel)]="foo">)?
3. Are you using a custom form control declared inside an NgModule? if so, make sure you are importing the NgModule.
4. Are you using ngModel with a third-party custom form control? Check whether that control provides a value accessor. If not, use ngDefaultControl on the control's element.
5. Are you testing a custom form control? Be sure to configure your testbed to know about the control. You can do so with Testbed.configureTestingModule.
6. Are you using Nx and Module Federation with Webpack? Your webpack.config.js may require extra configuration to ensure the forms package is shared.