Webextension is corrupted

When I try to install the webextension from Mozilla's add-ons website, it fails with the following report:

Error: Installation aborted because the add-on appears to be corrupt.

Likewise, when I compile a webextension from master, change the file extension from ‘.zip’ to ‘.xpi’ and try to install that, I get a similar message from Icecat (Firefox):

Error: This add-on could not be installed because it appears to be corrupt.

Web browser: GNU Icecat 60.7.0
Operating system: Parabola GNU+Linux-libre