I just tried defense mode and died in wave 1.
After that, I started a normal game.
The first thing the game tells me after character creation is the title of this bug report.
Perhaps related (not cleaning up properly after char death), is the tutorial:
After I ignited the gas pump and blew myself up, the game saved that status for the tutorial, and consequent tutorial characters will find themselves in a blazing inferno.
EDIT: the game really thinks I am still in defense mode, for when I try to save and exit, it tells me that I cannot save in defense mode, even though I started a normal game.
EDIT: when I suicided, it told me that I have survived wave 2 !. Come to think of it, I think the game congratulated me on surviving wave 1 as well, when I initially died in defense mode.