arguments are not toastable

Hi,
I have defined some function and also used NDBOX structure that having
variable length.

typedef struct NDBOX
{
int32 vl_len_; /* varlena length */
unsigned int dim;
double x[1];
} NDBOX;

When i called my function, it gives NDBOX to be null
On debugging, i found out ,FunctionInvokeCall invokes fmgr_oldstyle
function, for getting argument

if (fnextra->arg_toastable[i]) //this returns false, not able to get
arguments
fcinfo->arg[i] =
PointerGetDatum(PG_DETOAST_DATUM(fcinfo->arg[i]));
}

Why it is going into fmgr_oldstyle if the arguments is not toasted? "How to
get arguments toastable??" and even my table pg_class.reltoastrelid entry is
zero.

Thanks