错误：无法解析CustomTranslateLoader的所有参数... json导入问题？

我使用的是Translate，我必须编写自己的加载器，因为默认加载器基于httpClient。我的解决方案有效，项目启动并且语言更改。所以一切都很好。不幸的是，当我尝试构建时ng build my-lib --prod出现错误。我想这和json有关吗？（角度9）。

custom-loader.ts

import { TranslateLoader } from '@ngx-translate/core';
import { Observable, of } from 'rxjs';
import * as en from '../../../lib-assets/i18n/en.json';
import * as pl from '../../../lib-assets/i18n/pl.json';

export class CustomTranslateLoader implements TranslateLoader {
  constructor(private availableLang: any = { en, pl }) {}
  getTranslation(lang: string): Observable<any> {
    return of(this.availableLang[lang].default);
  }
}

tsconfig.json

"resolveJsonModule": true,
"esModuleInterop": true,
"allowSyntheticDefaultImports": true,

之后的错误 ng build my-lib --prod：

ERROR: Can't resolve all parameters for CustomTranslateLoader in /Projects/my-app/projects/my-lib/src/lib/core/translate/custom-loader.ts: (?).

An unhandled exception occurred: Can't resolve all parameters for CustomTranslateLoader in /Projects/my-app/projects/my-lib/src/lib/core/translate/custom-loader.ts: (?).