'_Fill' error when copying sheet?

I get the attached '_Fill' error when I attempt to copy one of my worksheets. It does allow me to rename it (whatever "it" is) but I'd really like to determine the reason for this error and eliminate it if possible (I'm going to be making a lot of copies of this sheet).

I cannot find a formula or named range that contains "_Fill" so I have no idea what it it is referring to.

Anybody point me in the direction of what is causing this?

Thanks!
Chris