appstreamcli not able to explain correctly a wrong icon tag

[fedelibre]

As discussed here, appstreamcli validate was not able to explain a problem in a metainfo file. Even worse, it was leading me on a wrong track.

The error printed by appstreamcli is:

$ appstreamcli validate-tree --no-net --explain build/files
org.frescobaldi.Frescobaldi.metainfo.xml
  E: org.frescobaldi.Frescobaldi:~: app-categories-missing
     This component is in no valid categories, even though it should be. Please check its metainfo
     file and desktop-entry file.

? Validation failed: errors: 1, pedantic: 1

The categories are set in the desktop file and I know that appstreamcli should check categories in the desktop file if they are not present in the metainfo. So my guess was that for some reason the desktop file could not be found. A wrong guess.

Eventually I was told that the problem is the presence of this tag in my metainfo file:

<icon type="stock">org.frescobaldi.Frescobaldi</icon>

For the records, appstream-util was able to detect and explain the error correctly, here's the output:

• tag-invalid           : stock icon is not valid [org.frescobaldi.Frescobaldi]

Can you improve this error detection?
Thanks