Hi,
Thank you for submitting a bug report. I have reproduced this issue and logged a report for it here:
You can track the report’s status as the issue is reviewed by our development staff. I also noted that I wasn’t able to reproduce this issue when I already applied a material to all of the material fields to the asset prior to the merging process. If you do get a crash try importing the asset fresh and applying the materials then merge.
Hope that helps,