I have two virtually identical 1041 tax returns.   Both are "complex trusts"   Both are small trust, with very little dividend income, but the income was not distributed or paid out to the named beneficiaries.  So, the dividend and interest income are taxable at trust level.  There is no distribution deduction.   I have entered the beneficiary information for the single beneficiary for each separate trust. 

Yesterday, for what I will call Trust A, there is $1,023 of income, there is a $100 exemption amount on line 21.  LIne 23 has taxable income of $923, and line 24 has total tax of $27.

Today I am completing another Form 1041 for another client, call this one Trust B.   Everything is essentially the same in all parameters.   this one has $854 of income, a $100 exemption, and $754 of taxable income on line 23.   Line 24, total tax is zero.   How can the total tax be zero on this line.  I cannot for the life of me figure out what is wrong here?  Or what I have inadverently entered differently.   Help.