Access violation writing location 0x00000004.

Hi all
I have this code that appears to be simple enough

Code:

#define TEST_LIMIT 100
        .
        .
        .

struct menu_info {        
        long rpt_options;        
        long rpt_default;        
        char **narr_menu;        
        char **plot_menu;        
        char **eqns_menu;        
        char **cust_menu;        
};
        .
        .
        .

        struct menu_info *M[TEST_LIMIT];
        .
        .
        .

void main(int argc, char **argv)
{
        .
        .
        .

        for(i = 0; i < TEST_LIMIT; i++)
                M[i]->rpt_default = 0L;

on the first iteration (i.e i=0) I get this message:
Unhandled exception at 0x013d3406 in myprogram.exe: 0xC0000005: Access violation writing location 0x00000004.

Before getting into the for loop, M appears to be correct and pointing to where it supposed to.

Any clue anybody?
Thanks
ZA