Problem task 168

i did the code and i think it works good, i think it do what the problem expect to do, but when i submit it the expected answer is "Error: Data pointer is decremented below zero with "<" operation" i don´t understand what does it means with HERE IS A SAMPLE TEXT TO BE ENCRYPTED BUT YOUR CODE WILL BE TESTED ON ANOTHER ONE. the answer is IFSF JT B TBNQMF UFYU UP CF FODSZQUFE CVU ZPVS DPEF XJMM CF UFTUFE PO BOPUIFS POF me code is

#include <stdio.h>
char cadena[100]={"0"};
char alfa[26]={'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
char encry[100];
int main (void)
{
int i=0;
while(cadena[i]!='.')
{
    i++;
    scanf("%c",&cadena[i]);
    for(int k=0; k<26; k++)
    {
        if(cadena[i]==alfa[k]) encry[i]=alfa[k+1];
        else if(cadena[i]==32) encry[i]=32;
        else if(cadena[i]=='Z') encry[i]=alfa[0];
    }
}
for(int j=1; j<i;j++)
{
    printf("%c",encry[j]);
}
}