Montana credit for income tax paid another state

I think this is a TT issue on Montana Form 2, page 9-22023, "Credit for income tax paid to another state...". I am blocked by the review process based on Line 4 of this form. The error message reads "Oth State Cred: Tot Inc Tax Liability 2nd must be less than or equal to zero" The number input is the actual tax paid and that number is used down further to calculate the actual credit. How can the number be less than or equal to zero?

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