Char** conditional jump or move depends on uninitialised value

char **alphabet = malloc(sizeof (char) * 100); 

#define NBCHAINES 26  
#define NBCARACTERES 10  
char **alphabet = malloc(sizeof (char *) * NBCHAINES);   
int k;  
for (k=0; k<NBCHAINES; k++) {  
  alphabet[k]=malloc(sizeof(char) * NBCARACTERES);  
  alphabet[k][0]='a'+k;  
}  

void free_alphabet(char **alph)
{
   int i = 0;
   while (alph[i] != NULL)
   {
       free(alph[i]);
       i++;
   }
   free(alph);
}

int main (void)
{
char **alphabet = malloc(sizeof (char) * 100); 
int i = 26; 
int nb = 0; 

while (i >= 0) 
{ 
    alphabet[nb][0] = 'a' + nb; 
    nb++; 
    i--; 
} 

free_alphabet(alphabet);

return (0);
}