MN State Income Using Fed AGI Even Though Some Income Was Earned in Another State

My wife and I live in Wisconsin and file as Married Filing Jointly.  I work in Minnesota while she works in Wisconsin.  When filing MN state taxes, Turbotax starts by using our federal taxable income which includes our joint income from both states but it never subtracts out the portion of our total earnings that was earned in Wisconsin.  Is this correct?  It seems questionable that our (my) Minnesota Taxable Income is greater than the gross amount of money earned in Minnesota?