Yeah, exactly. And since there's no way to give the unit defense that applies to missiles but not melee except for the shield, if you make the shield negative there is absolutely no way to make the unit's missile defense go back to being correct, at least not without adding extra melee defense TOO due to upping the armor stat. So while a negative shield value could theoretically make the unit work right in melee, it leaves no recourse for fixing the missile defense that it will have broken. Coincidentally, several people reported in this thread that a negative value won't stick for the shield anyway, the game sets anything negative to zero instead.