Copy RAM to flash strange error code

I am trying to write some data to Flash. While Prepare sector (cmd 50) and Erase sector (cmd 52) returns CMD_SUCCESS, I am having a hard time figuring out what should I send as a parameters to command 51.

  uint8_t data[] = {0x12, 0x88};
  uint8_t *dataPtr = &data[0];
  command[0] = 51;
  command[1] = a_dest; //destination address 0x1ff0
  command[2] = (uint32_t)dataPtr;
  command[3] = 2;
  command[4] = 48000000UL/1000; //frequency in kHz
  iap_entry(command, output);

This command returns output[0] == 0x0D, which doesnt even exist in datasheet as an error code...

While datasheet says that param 3 needs to be 256/512/1024 bytes long, I am not sure what should I do if I only want to update 2 bytes? Do I need first to read 256 bytes from (0x1ff2-256) and then change last two bytes in buffer and send that as a param2 ?

Microcontroller is LPC11C14.

Thanks