Mysterious Bus Error with get_fn_expr_argtype()

Howdy,

I'm trying to write a simple function that will return a string with
the type name of a value. Unfortunately, it keeps dying on me. I don't
even get any useful debugging information with --enable-cassert, just
this:

LOG: server process (PID 96946) was terminated by signal 10: Bus error
LOG: terminating any other active server processes
LOG: all server processes terminated; reinitializing

I stuck in a few calls to elog(), and it looks like this is the line
that's choking:

typeoid = get_fn_expr_argtype(fcinfo->flinfo, 0);

But that's copied directly from enum.c. So I'm pretty mystified. Any
help would be greatly appreciated.

Here's the complete code:

#include "postgres.h"
#include "fmgr.h"
#include "utils/builtins.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

extern Datum type_of (PG_FUNCTION_ARGS);

Datum
type_of(PG_FUNCTION_ARGS)
{
Oid typeoid;
Datum result;
char *typename;

typeoid = get_fn_expr_argtype(fcinfo->flinfo, 0);
if (typeoid == InvalidOid) {
ereport(
ERROR, (
errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("could not determine data type of argument to
type_of()")
)
);
}

typename = format_type_be(typeoid);
result = DirectFunctionCall1(textin, CStringGetDatum(typename));
PG_RETURN_DATUM(result);
}

And I load the function like so:

CREATE OR REPLACE FUNCTION type_of(anyelement)
RETURNS text
AS '$libdir/type_of'
LANGUAGE C STRICT IMMUTABLE;

Thanks,

DAvid