I can't seem to figure out how to use this plugin, because when I have two files open in two buffers, I try to compare them with diff-buffers anotherNamedBuffer, I get this error:

to rewrite a part of it: eval ... diff-update-marks ... cannot overwrite existing file without -force
(sorry for the screenshot, I can copypaste out of the terminal)
when I use the switch, it gives me an error about incorrent number of arguments.
Anything I'm doing wrong, or is it a fault on the plugin's side?
Thanks!