Problem with bus in a s-function

I have a hand-written C S-Function, which compiles but when I execute it, it gives the following error:

" 'Proba1/SFunction' cannot modify the zero representation of data type 'BALISEDATA_MESSAGE'. "

I think my problem can be in the following piece of code:

#ifndef __SL_DTYPE_BALISEDATA_MSG_C__
#define __SL_DTYPE_BALISEDATA_MSG_C__

/* include *******************************************************************/

#include "simstruc.h"
#include "BaliseData_message.h"

DTypeId Register_DType_BALISEDATA_Msg(  SimStruct *S )
{
  /* Define variables */
  int_T        status;
  DTypeId      id;
  BALISEDATA_DATATYPE tmp;

  /* Register the user-defined data types */
  id = ssRegisterDataType(S, "BALISEDATA_MESSAGE");
  if(id != INVALID_DTYPE_ID)
  {
    /* Set the size of the user-defined data type */
    status = ssSetDataTypeSize(S, id, sizeof(BALISEDATA_DATATYPE));
    if(status == 0)
    {
      id = INVALID_DTYPE_ID;
    }
  }
  if(id != INVALID_DTYPE_ID)
  {
    /* Set the zero representation */
    uint8_T  byteIndex;

    tmp.New_Balisse      = (boolean_T)0;
    tmp.BalissePosition    = (real32_T)0;
    tmp.GroundInclination  = (real32_T)0;
    tmp.NextBalissePosition  = (real32_T)0;

    status = ssSetDataTypeZero(S, id, &tmp);
  }
  return(id);
}

#endif

Can anybody help me?

[Kaustubha Govind]

Perhaps the zero representation of the type had already been set to a different value while you were trying this out? Look at the value returned by ssGetDataTypeZero and try doing a memcmp to the value you are trying to set (tmp).