Cannot drag and drop to archives with custom extension format

[dreadbalaur]

This message keeps popping up when I try to drag and drop into an archive with a custom program-specific extension. This is the case for dragging and dropping from Windows Explorer and between archives Despite the custom extension format, these archives are just .zip files. Drag and drop performs normally if I rename the archives to .zip, but that is an unnecessary extra step to take.
The files are not used by any other processes except for Nanazip, as also confirmed by File Locksmith. For reference, 7zip has been able to handle .zip with custom extensions for ~20 years.