Crash when parsing multi-cd album

[classical11]

Previously I had no album artist set for this album.

If I assign an album artist, it is not crashing any more. This is a little artificial since the album doesn't have a single overall artist but it's a workaround.

I also found that in some situations when I don't assign an album artist for a multi-CD album, the CDs are not merged correctly. Is the album artist required for multi-CD albums which have different artists?

I followed the instructions in the user guide which indicate that the requirements for merging multiple CDs are setting the DISCNUMBER and having an identical ALBUM title for all CDs:
http://minimserver.com/ug-browsing.html#Multi-disc albums