Writing in flash memory sector-0 of LPC 2144

Dear Sir,
I am trying to write in sector-0 of flsh memory of LPC 2144. The code gets compiled and hex file is generated successfully. But when I read from it, currupt data is obtained. Is there any extra precaution required in writing to sector-0 or is it allowed or not to write in sector -0. However I am able to read and write in other sectors of the same successfully. Following code is used:
#define IAP_LOCATION 0x7FFFFFF1

char Ram_Array[] = "TEST WITHOUT PEN DRIVE SEC. 0 ";

unsigned long command[5]; // For Command Table

unsigned long result[2]; // For Result Table

typedef void (*IAP)(unsigned long [], unsigned long []);

IAP iap_entry;

void main ()
{

unsigned long i,j;
char a1,a2,a3,a4;
char *FlashPointer;
char *Ram_Pointer;

InitSystem();
PINSEL0 = 0x00050000 ;
iap_entry = (IAP)IAP_LOCATION;// Set Function Pointer
Ram_Pointer = (char*)0x40000100; // Set pointer to RAM
for (i=0;i<256;i++)
{ if(i<=30) *Ram_Pointer = Ram_Array[i];
else *Ram_Pointer = '0';
Ram_Pointer++;
}

Write_to_Flash(1,1,256,0x1000);

while(1);

}

void Write_to_Flash(unsigned char Start_Sector, unsigned End_Sector, int No_of_Bytes, unsigned long Mem_Addr)
{ command[0] = 50; // Prepare sector(s) for a Erase Operation
command[1] = 0;
command[2] = 0;
iap_entry(command,result);

command[0] = 52;
command[1] = 0;
command[2] = 0;
command[3] = 48000; // PCLK == 48000 KHz
iap_entry(command,result);

command[0] = 50;
command[1] = 0;
command[2] = 0;
iap_entry(command,result);

command[0] = 51;
command[1] = 0x00000000;
command[2] = 0x40000100;
command[3] = 256;
command[4] = 48000;// PCLK = 48000 KHz
iap_entry(command,result);
}

Kindly suggest.

Vikas