invalid memory alloc after insert with c trigger function

Hello Everybody!

I know that something doing wrong, but I can't find out what is it. This is a part from trigger function:

attnum[3] = SPI_fnumber( tupdesc, "arfolyam" );
datums[3] = _selectFunctionB( "SELECT ertek FROM foo WHERE parameter='rate'" ); // this come back with Datum type from select - PG_RETURN_FLOAT8( b ); is end of this function where b is double type
arfolyam = DatumGetFloat8( datums[3] );
elog( INFO, "2.5.0 arfolyam = %f", arfolyam );
if ( attnum[3] == PointerGetDatum( NULL ) ) {
   elog( ERROR, "Hianyzo arfolyam" );
   SPI_finish();
   return PointerGetDatum(NULL);
}
elog( INFO, "2.5.1 Datums[3] = %f", DatumGetFloat8( datums[3] ) );
rettuple = SPI_modifytuple( trigdata->tg_relation, tmptuple, 4, &attnum[0], &datums[0], &isNull[0] );
SPI_freetuple(tmptuple);
SPI_finish();
return PointerGetDatum( rettuple );

After insert:
INFO: string vissza : 1.000000
INFO: 2.5.0 arfolyam = 1.000000
INFO: 2.5.1 Datums[3] = 1.000000
INSERT 0 1
ELES=# SELECT * FROM pgsor;
ERROR: invalid memory alloc request size 4294967293

The datums[0 - 2] are char* / VARCHAR type. If I set SPI_modifytuple( trigdata->tg_relation, tmptuple, 3, &attnum[0], &datums[0], &isNull[0] ); than everything is OK after insert.

The question: what is wrong with converting double to Datum?

Thanks in advance!
dj