Schedule A Medical Expenses Wrksht, line 2e long term care premium will accept an arithmetic negative final value (found this out by typo mistake, then tax due panic, then correction).  No error is listed by TT.  The "+" details list expansion for a tax form entry can typically accept negative list items but one would hope the list total should always be positive for most tax form line entries, especially Schedule A deductions.  In fact, much of the complexity of the US tax forms is to AVOID filers have to deal with negative numbers.  Every time you see the "enter the greater/lesser of" or  "enter zero if line X is greater/lesser than line Y" constructs it is to avoid locally recording then propagating an arithmetic negative.   An expected value add of TT is at least a warning if not an error for final tax form line entries where an arithmetic negative would never make sense.  Where the concept of a loss (business, capital, assets, etc.)  is a possibility, propagating a negative can make sense.  But accepting a negative deduction item? ... View more