delete user cannot be saved need free disk

I'm running Yosemite with all the latest updates and trying to delete a user account I created for a client. I have 43GB (out of 499) free on my boot drive. Yet when I try to perform this operation, "Save the home folder in a disk image" is not available for selection. It is greyed out and says "the home folder cannot be saved. To save the folder you need 4.4 GB of free disk space".

The other options are permitted, "don't change the home folder" and "delete the home folder", so I would think permissions are correct (I authenticated as an admin when I un-clicked the lock on the users and groups panel).

Why is this operation not permitted, when I'm an admin, and clearly have available disk space? I want to save the folder so I can put it on a backup disk.