Division by 0 in TurboPascal7

I am working on my school project and I am (almost) furious about this issue. My solution's validity is 95 %, but I have 1 input wrong – I don't know the input, but I know, that the input failed because of the error code 200, which is divison by 0.

My project is this:

„The digit sum of an integer is defined to be the sum of the digits in the integer's written form. Usually we calculate the digit sum using a number's decimal (base 10) representation, but in this exercise we will compute it in a given base. Write a function that computes a natural number's digit sum when the number is written in a given base. Using this function, write a program that reads two natural numbers (each on its own line, and both in decimal representation): a base B and a number N. The output should be N's digit sum in base B (written in decimal representation).“

I have tried to fix the problem by adding 2 conditions, but it did nothing, mistake still exists. I am using a formula, which can be found here https://en.wikipedia.org/wiki/Digit_sum . Here is the code itself, it is not long.

program CifernySoucetZobecneny;

var
  Soustava, Cislo, i: longint;
  HorniMez: longint;
  Soucet: real;

function Mocnina(Zaklad: longint; Exponent: longint): longint;
var
  i, Pomoc: longint;
begin
  Pomoc := 1;

  for i := 1 to Exponent do
  begin
    Pomoc := Pomoc * Zaklad;
  end;

  Mocnina := Pomoc;
end;

begin
  readLn(Soustava);
  readLn(Cislo);

  if (Soustava = 0) then
  begin
    writeLn('0');
    exit;
  end;

  if (Soustava = 1) then
  begin
    writeLn(Cislo);
    exit;
  end;

  HorniMez := Trunc(Ln(Cislo)/Ln(Soustava));
  Soucet := 0;

  for i := 0 to HorniMez do
  begin
    Soucet := Soucet + ((1/Mocnina(Soustava, (i)))*((Cislo mod Mocnina(Soustava, (i+1))) - (Cislo mod Mocnina(Soustava, i))));
  end;

  writeLn(Soucet:0:0);
end.

I would be grateful if someone could look at the code and told me, where am I dividing by 0, because I tried many inputs, spend many hours on this and did not come with the correct solution. Thank you all.

PS: I am Czech, so the names of the variables and functions are not in English, sorry for that, but I hope, that it will not be a problem.

When Cislo = 0, a following calculation of Ln(Cislo) gives the runtime error 200. That is because Ln(0) is undefined (outside the allowed range of Ln()).