Installing CM13 Nightly as MultiRom in TWRP fails
Try this workaround:
http://forum.xda-developers.com/showpost.php?p=63431628&amp;postcount=50

"Delete all the lines after "ui_print("Verifying the updated system image...");" until "show_progress(0.020000, 10);". once you have done it save the file and copy it back into the location inside the rom zip file and flash it on your phone."

Worked for me.