When I dump a font generated with Glyphs 2.5b (1066), there are some error messages about empty CMAP subtables:
ERROR: cmap subtable is reported as having zero length: platformID 0, platEncID 4, format 0 offset 3408. Skipping table.
ERROR: cmap subtable is reported as having zero length: platformID 3, platEncID 10, format 0 offset 3408. Skipping table.
Dumping 'cmap' table...