Why is my QBI deduction limited?

Why is my QBI deduction limited to "20% of taxable income net of capital gain" if there is no capital gain?

 

Why not the "20% of QBI from this business", since there is no capital gain?

 

Is this a bug in TurboTax logic?  Or is there something I don't understand?  If so, please explain.

 

Thank you.