Problem parsing ISRC data

I'm trying to write a web source script that fetches ISRC data and apply them to tracks, but encountered difficulty in doing so. Although track titles do apply cleanly to AAC files, for some unknown reason mp3tag is treating ISRC as global tag which should be applied to whole album (like PUBLISHER etc).

But since ISRC format was wrong (it was becoming ISRC for all tracks concatenated with separator), the net result is, the ISRC tag with literal string "" was added to all tracks.

What have I done wrong in this case? A (non-working) script is attached.

DeccaClassics.src (2.02 KB)