Merge-files tool error message

First of all, on behalf of the community, thank you for the new merge tool! I did some tests with it, sometimes with error messages, but it generated the output of merged.megan file(s).
But now with a brand new datasets it stopped with the following error message:

./merge-files -i N91-01_long.daa N91-01_short.daa -o N91-1-merged -v

MergeFiles - Computes the comparison of multiple megan, RMA or meganized DAA files
Options:
Input and Output:
–in: N91-01_long.daa N91-01_short.daa
–out: N91-1-merged
Other:
–verbose: true

Version MEGAN Community Edition (version 6.23.2, built 9 May 2022)
Author(s) Daniel H. Huson
Copyright © 2022 Daniel H. Huson. This program comes with ABSOLUTELY NO WARRANTY.

Input file: N91-01_long.daa
108 reads
Input file: N91-01_short.daa
939,119 reads

Caught:
java.lang.NullPointerException: Cannot invoke “megan.data.IClassificationBlock.getKeySet()” because “classificationBlock” is null
at megan/megan.data.merge.MergeClassificationBlock.getKeySet(MergeClassificationBlock.java:83)
at megan/megan.core.SyncArchiveAndDataTable.syncClassificationBlock2Summary(SyncArchiveAndDataTable.java:121)
at megan/megan.core.SyncArchiveAndDataTable.syncArchive2Summary(SyncArchiveAndDataTable.java:107)
at megan/megan.tools.MergeFiles.run(MergeFiles.java:113)
at megan/megan.tools.MergeFiles.main(MergeFiles.java:49)

I wonder what is the reason for this? ( I can open the short/long daa files separately, everything is fine with them)

I’m looking at the code and can’t figure out what the problem is…
If you can give me access to two files that cause this problem, then that would be very hepful!