Before using this formula

PV*(1+r)^n + pmt*((1+r)^n - (1+i)^n))/(r-i) + FV = 0

you should check for r==i, to avoid division by 0. If r==i, use

PV*(1+r)^n + pmt*n*(1+r)^(n-1) + FV = 0