a question about form 6251 (AMT tax return)

[asktax]

I am a little confused when I filled the from 6251. It seems line 39 is the key line for the long term capital gain calculation. If the line 10 of my Schedule D Tax Worksheet is empty, should I fill "0" in line 39? If line 39 is zero, line 40 will be zero and line 41 will be equal to line 36. As a result, the tax rate of my long term capital gain will not be calcalated as 15% and it will be treated as the same of ordinary income.

Any help on how to fill line 39 will be greatly appreciated!

Thanks again,
asktax