Topographic Wetness Index (TWI) / Compund Topographic Index (CTI)

Dear Group,

I am trying to calculate TWI/CTI by using these formulas (based on what I have read so far):

Equation 1: TWI = ln ((As+1)/tanB)
Equation 2: TWI = ln ((As+1 * cell size)/tan B)

where, As = flow accumulation
B = slope in radian, I have also converted zeros to a small value of 0.000001.
cell size is 90 meters

The question is that: can TWI results be a negative value? Equation 1 produces negative values whereas Equation 2 doesn't. Not sure which one to use. And I don't know why Equation 1 produces negative value.

Thank you in advance for your help.

Risa