Negative DNI estimates from DISC model

Hello,
I am attempting to wrap pvwattsv5 into my own program for estimating solar output over specific historical periods. I have all most necessary weather data (GHI, Tdry, Wspd) for input to the module. However, when trying to estimate DNI from GHI using the DISC model, many negative values are being output. The only changes that I have made to the DISC model have been extending equations out to 8760 hours, adding a zero hour to Jan 1, and changing default inputs to inputs from one of my own stations (near Albany, NY) - the file is attached. Any insight you can give me into where I have made mistakes would be greatly appreciated.