Problem with Numerics multiplication in C-function

Hi, I have a strange problem with Numeric multiplication in C-function.

There are 2 functions getRate and getVAT which return Numeric. In 3rd
function calculateService I try to multiply the results of getRate and
getVAT. After execution I have two types of error, some time without
messages and with message "Invalid memory alloc ... ".
If I initialize the Numeric variables inside calculateService and
multiply, function numeric_mul works fine.

PostgreSQL 8.3.7

Datum getRate(PG_FUNCTION_ARGS) {
int32 accountId = PG_GETARG_INT32(0);
int16 serviceId = PG_GETARG_INT16(1);
DateADT date = PG_GETARG_DATEADT(2);

bool isNull;
char command[QUERY_MAX_SIZE];
char message[MESSAGE_MAX_SIZE];

Numeric rate = DatumGetNumeric(DirectFunctionCall3(numeric_in,
CStringGetDatum(RATE_ERROR_CSTRING_VALUE), 0, -1));

//Build SQL query
snprintf(command, sizeof (command), "...");

SPI_connect();
SPI_execute(command, true, 0);
rate = DatumGetNumeric(SPI_getbinval(SPI_tuptable->vals[0],
SPI_tuptable->tupdesc,
1,
&isNull));

#ifdef PR_DEBUG
snprintf(message, sizeof (message), "<DEBUG> getRate: Returns rate =
%s.",
DatumGetCString(DirectFunctionCall1(numeric_out,
NumericGetDatum(rate))));
elog(INFO, message);
#endif
SPI_finish();
PG_RETURN_NUMERIC(rate);
}

Datum calculateService(PG_FUNCTION_ARGS) {
// Like the getRate
}

Datum calculateService(PG_FUNCTION_ARGS) {
int32 accountId = PG_GETARG_INT32(0);
int16 serviceId = PG_GETARG_INT16(1);
DateADT date = PG_GETARG_DATEADT(2);
int32 transactionRegisterId = PG_GETARG_INT32(3);

Numeric rate;
Numeric vat;
Numeric amount;

rate = DatumGetNumeric(DirectFunctionCall3(getRate,
Int32GetDatum(accountId),
Int16GetDatum(serviceId),
DateADTGetDatum(date)));
vat = DatumGetNumeric(DirectFunctionCall1(getVAT,
DateADTGetDatum(date)));

#ifdef PR_DEBUG
snprintf(message, sizeof (message), "<DEBUG> calculateService: rate
= %s, vat",
DatumGetCString(DirectFunctionCall1(numeric_out,
NumericGetDatum(rate))));
elog(INFO, message);
#endif

amount = DatumGetNumeric(DirectFunctionCall2(numeric_mul,
NumericGetDatum(rate),
NumericGetDatum(vat)));

// ERROR
...
}

Best regards,
Ilya Urikh.