I too had negative AGI in 2017, and TT wouldn't accept a negative entry, and so I changed it to $0 as instructed. Unfortunately, even though TurboTax accepted it, the IRS did not, and my extension was rejected. This all happened in the 11th hour, and I wasn't aware of the rejection until after midnight on 4/15/2019, so I missed the filing deadline and had to file a paper extension this morning, 4/16/2019. Pretty disappointing that as powerful as Turbotax is, you could get tripped up by such a simple flaw in the system.
