polymer build -> Error: file path is not in root

[hyptes...@gmail.com]

Hello,
my Polymer app works as expected with polymer serve.
However when I try to build it goes wrong:

error:   Promise rejection: Error: file path is not in root: O:\bower_components\promise-polyfill\Promise.js (O:\Samos_Frontend)
error:   Error: file path is not in root: O:\bower_components\promise-polyfill\Promise.js (O:\Samos_Frontend)

This makes no sense to me. When the app is running, how it is not able to build?
I googled about it. Some gave an hint with a polymer.json file. Explanation for that is very poor. It did not help me.

Can anybody please be precisely about that?

S.