I install ngx-persian package in my angular app root: npm install ngx-persian --save. once I import faNumPipe in the declarations array of the @NgModule of my module, I get errors such that all components declared in my module are unknown! and also this error: NG6001: Cannot declare 'FaNumPipe' in an NgModule as it's not a part of the current compilation. I followed instructions of https://www.npmjs.com/package/ngx-persian#how-to-install correctly to install and import the pipes of ngx-persian. when I remove the imported pipes from module, compilation is done successfully. where is the problem and how fix it? best regards.