Bug in AdjustIntervalForTypmod function

PostgreSQL 7.3.2 on i686-pc-linux-gnu, compiled by GCC 2.96

The following function definition:

extern Datum testDate(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(testDate);

Datum
testDate(PG_FUNCTION_ARGS)
{
Datum tnow = DirectFunctionCall3(timestamp_in,
CStringGetDatum("now"),
ObjectIdGetDatum(InvalidOid),
Int32GetDatum(0));

Datum interv = DirectFunctionCall3(interval_in,
CStringGetDatum("7 Days"),
ObjectIdGetDatum(InvalidOid),
Int32GetDatum(0));

Datum tnow_pl_interval = DirectFunctionCall2(timestamp_pl_span,
tnow,
interv);

text *ptext = DatumGetTextP(DirectFunctionCall1(textin, DirectFunctionCall1(timestamp_out,tnow_pl_interval)));

if (ptext == NULL)
PG_RETURN_NULL();
PG_RETURN_TEXT_P(ptext);
}

When called such as:
Bugs=> SELECT "testDate"();

Produces this error message:
ERROR: AdjustIntervalForTypmod(): internal coding error

Yet when I call the equivalent SQL it reports no error, such as:
Bugs=> SELECT timestamp_pl_span('now'::timestamp(0), '7 Days'::interval(0));
timestamp_pl_span
---------------------
2003-05-20 17:35:21
(1 row)

I searched through the code and found that the failure was occuring in the function:
"static void AdjustIntervalForTypmod(Interval *interval, int32 typmod)"

To get around the problem I set the type modifier for the "Interval" as "-1".

Regards
Donald Fraser