code i correct why am i getting '++' needs l-value

error C2105: '++' needs l-value

sum = sum + *((word16 *) addr)++;

#include <stdio.h>                  // Needed for printf()
#include <stdlib.h>                 // Needed for rand()

//----- Type defines ----------------------------------------------------------
typedef unsigned char      byte;    // Byte is a char
typedef unsigned short int word16;  // 16-bit word is a short int
typedef unsigned int       word32;  // 32-bit word is an int

//----- Defines ---------------------------------------------------------------
#define BUFFER_LEN        6      // Length of buffer
//extern char data[6];
char data[6]="CM00";
//----- Prototypes ------------------------------------------------------------
word16 checksum(char *addr, word32 count);
 
//===== Main program ==========================================================
int tarun(void)
{
  //byte        buff[BUFFER_LEN]; // Buffer of packet bytes

  word16      check;            // 16-bit checksum value
  word32      i;                // Loop counter

  // Load buffer with BUFFER_LEN random bytes
  for (i=0; i<BUFFER_LEN; i++)
  {
    //buff[i] = (byte) rand();
	data[i]=(byte) rand();
  }
 
  // Compute the 16-bit checksum
  //check = checksum(buff, BUFFER_LEN);
  check = checksum(data, BUFFER_LEN);
 
  // Output the checksum
  printf("checksum = %04X \n", check);
  
}
 
//=============================================================================
//=  Compute Internet Checksum for count bytes beginning at location addr     =
//=============================================================================
word16 checksum(char *addr, word32 count)
{
  register word32 sum = 0;
 
  // Main summing loop
  while(count > 1)
  {
    sum = sum + *((word16 *) addr)++;
    count = count - 2;
  }
 
  // Add left-over byte, if any
  if (count > 0)
    sum = sum + *((byte *) addr);
 
  // Fold 32-bit sum to 16 bits
  while (sum>>16)
    sum = (sum & 0xFFFF) + (sum >> 16);
 
  return(~sum);
}