Webpack alias with Angular - Cannot use namespace 'UtilsService' as a type

I have a project with Angular 6, Typescript, webpack 4.

I want to use alias in webpack in order to make my import statement clearer.

This is the code I have in webpack:

resolve: {
  alias: {
    assets: `${helpers.root('src/assets/')}`,
    app: `${helpers.root('src/app/')}`
  },
}

The "assets" alias is working fine !

I use the "app" alias in this way within the code of a component:

import { UtilsService } from 'app/_services/utils.service';

And this is the constructor of the component:

constructor( @Inject( UtilsService ) private utils: UtilsService, ) {
}

I don't have an error on the import statement, so I guess that the alias works in some way.

But I have the following error in the constructor:

TS2709: Cannot use namespace 'UtilsService' as a type.

I must be missing some configuration somewhere but can't figure out what ?

Any help will be greatly appreciated

from Webpack alias with Angular - Cannot use namespace 'UtilsService' as a type