Debug :: I Get: Division By Zero

When I debug this script I get: division by zero!

In 3 lines over the debugging line, the script are writing out the variable "prosentref" as 1. Whatt can be wrong?

(the value in cell T8769 is 1)

Option Explicit
Sub optimaliseringDrift()
Dim Pgm1 As Integer
Dim Pgm2 As Integer
Dim PL As Integer
Dim lamda_gm1 As Double
Dim lamda_gm2 As Double
Dim deriv1 As Double
Dim deriv2 As Double
Const Pgm1_max As Integer = 200
Const Pgm2_max As Integer = 300
Const Pgm1_min As Integer = 45
Const Pgm2_min As Integer = 40
Const deltaP As Integer = 5