Ошибка сегментирования. Перевод числа в строковый эквивалент - C (СИ)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
 
void FormString(int num);
int razryad(int num);
char *ResultString(int numeric);
 
int main()
{
    int num;
    printf("Введите число: ");
    scanf("%d", &num);
    printf("Введено: %d \n", num);
    FormString(num);
    return 0;
}
 
void FormString(int num)
{
    char *Razryadi[] = { " ","thousands ", "millions ", "billions "};
    int raz, divisor, bufnum;
    char *s1=" ";
    char *ps=" ";
    raz = razryad(num);
    for (raz; raz > -1; raz--)
    {
        divisor = pow((double)1000, (double)raz);
        bufnum = num/divisor;
        s1= ResultString(bufnum);
        num = num%divisor;
        strcat(s1, Razryadi[raz]);
        printf("%s",s1);
    }
    printf("\n");
}
 
int razryad(int num)
{
    int raz;
    for (raz=0; num/1000 > 0; raz++)
        {
            num=num/1000;
        }
    return raz;
}
 
char *ResultString(int numeric)
{
    char *Hundreds[] = {" ", "hundreed "};
    char *Tens [] = {"","", "twenty ", "thirsty ", "fourty ", "fifty ", "sixty ", "seventy ", "eighty ", "ninety "};
    char *Ones [] = {"", "one ","two ","three ","four ","five ","six ","seven ","eight ","nine ","ten ", "eleven ","twelve ","thirsteen ",
    "fourteen ","fiveteen ","sixteen ","seventeen ","eighteen ","nineteen "};
    char *Minus [] = {"", "minus"};
    int hund, tens, ones;
    char *shund = " ";
    char *stens = " ";
    char *sones = " ";
    char *RString = " ";
    char *mins = " ";
    if (numeric < 0) {mins = Minus[1]; numeric= -numeric;}
 
        hund = numeric/100;
        numeric = numeric%100;
        tens = numeric/10;
        numeric = numeric%10;
        ones = numeric;
        if (tens ==1)
        {
            ones = ones + 10;
            tens = 0;
        }
        if (hund < 1)
        {
            shund = Ones[hund];
            strcat(shund, Hundreds[0]);
        }
        else
        {
            shund = Ones[hund];
            strcat(shund, Hundreds[1]);
        }
        stens = Tens[tens];
        sones = Ones[ones];
        strcpy(RString, mins);
        strcat(RString, shund);
        strcat(RString,stens);
        strcat(RString, sones);
 
    return RString;
}