Schedule C line 21f,  should show Non-Montana municipal income taxable to MT.  Instead, Proconnect enters the total tax-exempt income.  No amount of overrides or modifications will correct the number.  This issue creates a negative number in the Schedule C calculation.  No error message was issued, until I efiled the return & it was rejected.  I received this message:

Solution: Exempt interest income cannot be negative for electronic filing purposes. Please check your entries and resubmit.

Code: /ReturnState/ReturnDataState/SchCMTFID/MTAdjustedExemptInterestIncome

Proconnect software needs to be changed to input the correct amount on line 21f, or at least allow an override on line 21f.  An error message should have been generated before attempting to efile.  I spent several hours on the phone with support with no resolution.  I was finally told, I would have to figure this out myself.