The .CSO file format causing Play! (PS2 emulator) to crash on exit?

This is a strange one, the core seems to load .ISO’s fine and exit fine, but any attempt to open a .CSO file it will launch ok, then trying to exit Retroarch it just freezes… I tried different compression tools to compress the .ISO with same result. Anyone have any ideas? (using core version 0.30-967835c)