async problems?

Here is a test program which dies in the postgres runtime. I've simplified
the code as much as I can, and I cannot see where I'm doing anything wrong.

Has anybody had success with async mode? Am I doing something wrong here?
Or are there some problems with async operation that I don't know about?

(the larger program does this in an event loop... this non-event loop
program exhibits the same behavior.)

Many TIA,
Mark

#include <stdio.h>
#include <stdlib.h>

#include "libpq-fe.h"

char fetchstr[] = "FETCH 100 in cur";

int main()
{
PGconn* conn;
PGresult *res;
int rc;
int tot = 0;
int ntuples;

conn = PQconnectdb("host=planb dbname=planb");
printf("PQconnectdb %d\n", PQstatus(conn));

rc = PQsetnonblocking(conn, 1);
printf("PQsetnonblocking %d\n", rc);

res = PQexec(conn, "BEGIN");
printf("PQexec begin %s\n", PQresStatus(PQresultStatus(res)));
PQclear(res);

res = PQexec(conn,"DECLARE cur CURSOR FOR select * from assets limit 8888"); printf("PQexec declare %s\n", PQresStatus(PQresultStatus(res)));
PQclear(res);

rc = PQsendQuery(conn, fetchstr);
printf("PQsendQuery %d\n", rc);
PQclear(res);
PQflush(conn);

while (1) {
res = PQgetResult(conn);
printf("tot=%d res=%p\n", tot, res);

if (res == NULL) {
rc = PQsendQuery(conn, fetchstr);
printf("PQsendQuery %d\n", rc);
PQflush(conn);
}
else {
ntuples = PQntuples(res);
tot += ntuples;

if (ntuples == 0) {
printf("done: tot=%d\n", tot);
res = PQexec(conn, "CLOSE cur");
printf("PQexec close %s\n", PQresStatus(PQresultStatus(res)));
PQclear(res);
res = PQexec(conn, "END");
printf("PQexec end %s\n", PQresStatus(PQresultStatus(res)));
PQclear(res);
PQfinish(conn);
exit(0);
}
PQclear(res);

}
}
}

#0 chunk_alloc (ar_ptr=0x40178a00, nb=2056) at malloc.c:2983
#1 0x400c4108 in __libc_malloc (bytes=2048) at malloc.c:2811
#2 0x40021063 in pqResultAlloc (res=0x8053928, nBytes=6, isBinary=0 '\0')
at fe-exec.c:271
#3 0x4002b1fb in getAnotherTuple (conn=0x8049e50, msgLength=21)
at fe-protocol3.c:530
#4 0x4002a9e9 in pqParseInput3 (conn=0x8049e50) at fe-protocol3.c:271
#5 0x400221fe in parseInput (conn=0x8049e50) at fe-exec.c:998
#6 0x4002231c in PQgetResult (conn=0x8049e50) at fe-exec.c:1072
#7 0x080489bb in main () at n6.c:37
#8 0x40060627 in __libc_start_main (main=0x8048850 <main>, argc=1,
ubp_av=0xbfffdee4, init=0x8048618 <_init>, fini=0x8048b70 <_fini>,
rtld_fini=0x4000dcd4 <_dl_fini>, stack_end=0xbfffdedc)
at ../sysdeps/generic/libc-start.c:129