This error happened to me because I used the default keyword to export my component :

@Component({
  selector: 'lib-form',
  templateUrl: './form.component.html',
  styleUrls: ['./form.component.scss'],
})
export default class FormComponent implements OnInit {
    // ...
}

The use of this keyword was suggested by my Linter and allows to write imports as import FormComponent from './form.component'; instead of import { FormComponent } from './form.component';

However this does not seem to work well along public-api.ts. The solution for me was to remove the default keyword and change all imports.

I was struggling with the same issue today.

My prerequisites:

• I work on an Angular 11 library type project;
• I have added a new directive;
• I got an error as above when tried to add my directive to module exports.

Fix:

• I have added file export to index.ts file:

export * from './just/a/relative/path/to/the/directive/{{myDirectiveFile}}';

This error happens if any component is exported in NgModuleand not included in your public_api.ts, Angular 9 will throw an error now.

This error was not coming in Angular 8 but after upgrading to Angular 9 it started showing.

If you exported any service, module or component, etc in NgModule make sure to include them in public_api.ts or else angular 9 will throw error now.

Fix: add your component to the public_api.ts

export * from './lib/components/some-me/some-me.component';