New use of "unless"?

[Herb Stahlke]

This evening at a local family restaurant in a Central Indiana
small town, the server, a local woman in her late 30s, asked if my
chili was hot (spicy) enough.  When I said it was, she replied, "I
can't eat it like that unless I have heartburn all night."  My
wife and I disagreed on what she meant, so when she returned with
more coffee I said, "So you like your chili hot."  She responded,
"Oh no!  I can't eat it that way.  It does awful things to my
ulcer."

Is this a unique use of unless or has it been observed elsewhere?

Herb Stahlke