Additional Billing Field Creation – Enter default value given in the options.

I added an additional billing field months ago that allows people to pick where they heard about us. It’s all been working fine until I tried to add a new option today and when I click to save nothing happens, at the top in red it displays – “Enter default value given in the options.”

I tried deleting the option I just created and changing the text on one of the options currently in the working version and it displays the same error at the top and save doesn’t work.

Any suggestions on a fix?